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diff --git a/test/fuzz_mongocrypt.c b/test/fuzz_mongocrypt.c
new file mode 100644
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--- /dev/null
+++ b/test/fuzz_mongocrypt.c
@@ -0,0 +1,329 @@
+/*
+ * Copyright 2024-present MongoDB, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/*
+ * OSS-Fuzz fuzzing entry point for libmongocrypt.
+ *
+ * Exercises the libmongocrypt state machine by initializing contexts for
+ * various operations and feeding fuzzed data through each state transition.
+ */
+
+#include "mongocrypt.h"
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* MONGOCRYPT_KEY_LEN is 96 bytes (defined in mongocrypt-crypto-private.h). */
+#define FUZZ_KEY_LEN 96
+
+/* Maximum number of state transitions to prevent infinite loops. */
+#define MAX_STATE_TRANSITIONS 32
+
+/* Operation types selected by the first byte of fuzz input. */
+enum {
+    OP_ENCRYPT = 0,
+    OP_DECRYPT = 1,
+    OP_EXPLICIT_ENCRYPT = 2,
+    OP_EXPLICIT_DECRYPT = 3,
+    OP_DATAKEY = 4,
+    OP_REWRAP_MANY_DATAKEY = 5,
+    OP_EXPLICIT_ENCRYPT_EXPRESSION = 6,
+    OP_COUNT = 7,
+};
+
+/* Helper: consume bytes from the fuzz input. */
+static const uint8_t *fuzz_consume(const uint8_t **data, size_t *remaining, size_t n) {
+    if (*remaining < n) {
+        return NULL;
+    }
+    const uint8_t *ptr = *data;
+    *data += n;
+    *remaining -= n;
+    return ptr;
+}
+
+/* Helper: consume one byte. */
+static int fuzz_consume_byte(const uint8_t **data, size_t *remaining) {
+    const uint8_t *p = fuzz_consume(data, remaining, 1);
+    return p ? *p : -1;
+}
+
+/*
+ * Drive the context state machine, feeding fuzzed data at each state.
+ * Returns when the context reaches DONE, ERROR, or we run out of input.
+ */
+static void drive_ctx(mongocrypt_ctx_t *ctx, const uint8_t **data, size_t *remaining) {
+    mongocrypt_ctx_state_t state;
+    int transitions = 0;
+
+    while (transitions++ < MAX_STATE_TRANSITIONS) {
+        state = mongocrypt_ctx_state(ctx);
+
+        switch (state) {
+        case MONGOCRYPT_CTX_NEED_MONGO_COLLINFO_WITH_DB:
+        case MONGOCRYPT_CTX_NEED_MONGO_COLLINFO:
+        case MONGOCRYPT_CTX_NEED_MONGO_MARKINGS:
+        case MONGOCRYPT_CTX_NEED_MONGO_KEYS: {
+            /* Feed fuzzed BSON-like data as a mongo response. */
+            size_t feed_len = 0;
+            const uint8_t *len_bytes = fuzz_consume(data, remaining, 2);
+            if (!len_bytes) {
+                return;
+            }
+            feed_len = (size_t)len_bytes[0] | ((size_t)len_bytes[1] << 8);
+            if (feed_len > *remaining) {
+                feed_len = *remaining;
+            }
+            if (feed_len > 0) {
+                const uint8_t *feed_data = fuzz_consume(data, remaining, feed_len);
+                if (feed_data) {
+                    mongocrypt_binary_t *bin =
+                        mongocrypt_binary_new_from_data((uint8_t *)feed_data, (uint32_t)feed_len);
+                    if (bin) {
+                        mongocrypt_ctx_mongo_feed(ctx, bin);
+                        mongocrypt_binary_destroy(bin);
+                    }
+                }
+            }
+            mongocrypt_ctx_mongo_done(ctx);
+            break;
+        }
+
+        case MONGOCRYPT_CTX_NEED_KMS_CREDENTIALS: {
+            /* Feed fuzzed KMS provider credentials. */
+            size_t feed_len = 0;
+            const uint8_t *len_bytes = fuzz_consume(data, remaining, 2);
+            if (!len_bytes) {
+                return;
+            }
+            feed_len = (size_t)len_bytes[0] | ((size_t)len_bytes[1] << 8);
+            if (feed_len > *remaining) {
+                feed_len = *remaining;
+            }
+            if (feed_len > 0) {
+                const uint8_t *feed_data = fuzz_consume(data, remaining, feed_len);
+                if (feed_data) {
+                    mongocrypt_binary_t *bin =
+                        mongocrypt_binary_new_from_data((uint8_t *)feed_data, (uint32_t)feed_len);
+                    if (bin) {
+                        mongocrypt_ctx_provide_kms_providers(ctx, bin);
+                        mongocrypt_binary_destroy(bin);
+                    }
+                }
+            } else {
+                /* Provide empty doc to advance past this state. */
+                uint8_t empty_bson[] = {5, 0, 0, 0, 0}; /* empty BSON document */
+                mongocrypt_binary_t *bin = mongocrypt_binary_new_from_data(empty_bson, sizeof(empty_bson));
+                if (bin) {
+                    mongocrypt_ctx_provide_kms_providers(ctx, bin);
+                    mongocrypt_binary_destroy(bin);
+                }
+            }
+            break;
+        }
+
+        case MONGOCRYPT_CTX_NEED_KMS: {
+            /* Iterate KMS contexts and feed fuzzed response data. */
+            mongocrypt_kms_ctx_t *kms_ctx;
+            while ((kms_ctx = mongocrypt_ctx_next_kms_ctx(ctx)) != NULL) {
+                uint32_t bytes_needed = mongocrypt_kms_ctx_bytes_needed(kms_ctx);
+                if (bytes_needed == 0) {
+                    continue;
+                }
+                size_t feed_len = bytes_needed;
+                if (feed_len > *remaining) {
+                    feed_len = *remaining;
+                }
+                if (feed_len == 0) {
+                    return;
+                }
+                const uint8_t *feed_data = fuzz_consume(data, remaining, feed_len);
+                if (feed_data) {
+                    mongocrypt_binary_t *bin =
+                        mongocrypt_binary_new_from_data((uint8_t *)feed_data, (uint32_t)feed_len);
+                    if (bin) {
+                        mongocrypt_kms_ctx_feed(kms_ctx, bin);
+                        mongocrypt_binary_destroy(bin);
+                    }
+                }
+            }
+            mongocrypt_ctx_kms_done(ctx);
+            break;
+        }
+
+        case MONGOCRYPT_CTX_READY: {
+            mongocrypt_binary_t *out = mongocrypt_binary_new();
+            if (out) {
+                mongocrypt_ctx_finalize(ctx, out);
+                mongocrypt_binary_destroy(out);
+            }
+            break;
+        }
+
+        case MONGOCRYPT_CTX_DONE:
+        case MONGOCRYPT_CTX_ERROR:
+        default: return;
+        }
+    }
+}
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+    /*
+     * Need at least 1 byte for op selection + some data to feed.
+     * Trivially small inputs won't exercise meaningful code paths.
+     */
+    if (size < 5) {
+        return 0;
+    }
+
+    const uint8_t *cursor = data;
+    size_t remaining = size;
+
+    /* Consume first byte to select operation. */
+    int op = fuzz_consume_byte(&cursor, &remaining) % OP_COUNT;
+
+    /* Set up mongocrypt_t with a local KMS provider. */
+    mongocrypt_t *crypt = mongocrypt_new();
+    if (!crypt) {
+        return 0;
+    }
+
+    /* Configure local KMS provider with a fixed 96-byte key. */
+    uint8_t local_key[FUZZ_KEY_LEN];
+    memset(local_key, 0xAB, FUZZ_KEY_LEN);
+    mongocrypt_binary_t *key_bin = mongocrypt_binary_new_from_data(local_key, FUZZ_KEY_LEN);
+    if (!key_bin) {
+        mongocrypt_destroy(crypt);
+        return 0;
+    }
+    mongocrypt_setopt_kms_provider_local(crypt, key_bin);
+    mongocrypt_binary_destroy(key_bin);
+
+    if (!mongocrypt_init(crypt)) {
+        mongocrypt_destroy(crypt);
+        return 0;
+    }
+
+    mongocrypt_ctx_t *ctx = mongocrypt_ctx_new(crypt);
+    if (!ctx) {
+        mongocrypt_destroy(crypt);
+        return 0;
+    }
+
+    /* Create a binary view of remaining fuzz data for init calls. */
+    mongocrypt_binary_t *input_bin = NULL;
+    if (remaining > 0) {
+        input_bin = mongocrypt_binary_new_from_data((uint8_t *)cursor, (uint32_t)remaining);
+    }
+
+    bool init_ok = false;
+
+    switch (op) {
+    case OP_ENCRYPT:
+        if (input_bin) {
+            init_ok = mongocrypt_ctx_encrypt_init(ctx, "test", -1, input_bin);
+        }
+        break;
+
+    case OP_DECRYPT:
+        if (input_bin) {
+            init_ok = mongocrypt_ctx_decrypt_init(ctx, input_bin);
+        }
+        break;
+
+    case OP_EXPLICIT_ENCRYPT: {
+        /* Set a fixed key id (16-byte UUID). */
+        uint8_t key_id[16];
+        memset(key_id, 0x61, sizeof(key_id)); /* matches "YWFhYWFhYWFhYWFhYWFhYQ==" */
+        mongocrypt_binary_t *kid_bin = mongocrypt_binary_new_from_data(key_id, sizeof(key_id));
+        if (kid_bin) {
+            mongocrypt_ctx_setopt_key_id(ctx, kid_bin);
+            mongocrypt_binary_destroy(kid_bin);
+        }
+        mongocrypt_ctx_setopt_algorithm(ctx, "AEAD_AES_256_CBC_HMAC_SHA_512-Deterministic", -1);
+        if (input_bin) {
+            init_ok = mongocrypt_ctx_explicit_encrypt_init(ctx, input_bin);
+        }
+        break;
+    }
+
+    case OP_EXPLICIT_DECRYPT:
+        if (input_bin) {
+            init_ok = mongocrypt_ctx_explicit_decrypt_init(ctx, input_bin);
+        }
+        break;
+
+    case OP_DATAKEY: {
+        /* Set up key encryption key for local provider. */
+        uint8_t kek_bson[] = {
+            /* {"provider": "local"} as BSON */
+            0x1b, 0x00, 0x00, 0x00,                                     /* doc len = 27 */
+            0x02,                                                        /* type: string */
+            'p', 'r', 'o', 'v', 'i', 'd', 'e', 'r', 0x00,              /* key */
+            0x06, 0x00, 0x00, 0x00,                                      /* string len = 6 */
+            'l', 'o', 'c', 'a', 'l', 0x00,                              /* value */
+            0x00                                                         /* doc terminator */
+        };
+        mongocrypt_binary_t *kek_bin = mongocrypt_binary_new_from_data(kek_bson, sizeof(kek_bson));
+        if (kek_bin) {
+            mongocrypt_ctx_setopt_key_encryption_key(ctx, kek_bin);
+            mongocrypt_binary_destroy(kek_bin);
+        }
+        init_ok = mongocrypt_ctx_datakey_init(ctx);
+        break;
+    }
+
+    case OP_REWRAP_MANY_DATAKEY:
+        if (input_bin) {
+            init_ok = mongocrypt_ctx_rewrap_many_datakey_init(ctx, input_bin);
+        }
+        break;
+
+    case OP_EXPLICIT_ENCRYPT_EXPRESSION: {
+        uint8_t key_id[16];
+        memset(key_id, 0x61, sizeof(key_id));
+        mongocrypt_binary_t *kid_bin = mongocrypt_binary_new_from_data(key_id, sizeof(key_id));
+        if (kid_bin) {
+            mongocrypt_ctx_setopt_key_id(ctx, kid_bin);
+            mongocrypt_binary_destroy(kid_bin);
+        }
+        mongocrypt_ctx_setopt_algorithm(ctx, "AEAD_AES_256_CBC_HMAC_SHA_512-Deterministic", -1);
+        if (input_bin) {
+            init_ok = mongocrypt_ctx_explicit_encrypt_expression_init(ctx, input_bin);
+        }
+        break;
+    }
+
+    default: break;
+    }
+
+    if (input_bin) {
+        mongocrypt_binary_destroy(input_bin);
+    }
+
+    /* If initialization succeeded, drive the state machine with fuzzed data. */
+    if (init_ok) {
+        drive_ctx(ctx, &cursor, &remaining);
+    }
+
+    mongocrypt_ctx_destroy(ctx);
+    mongocrypt_destroy(crypt);
+
+    return 0;
+}
+