diff --git a/src/core/config/Categories.json b/src/core/config/Categories.json
index 20846692..a9d54d7b 100644
--- a/src/core/config/Categories.json
+++ b/src/core/config/Categories.json
@@ -244,6 +244,7 @@
             "Multiply",
             "Divide",
             "Extended GCD",
+            "Modular Inverse",
             "Mean",
             "Median",
             "Standard Deviation",
diff --git a/src/core/operations/ModularInverse.mjs b/src/core/operations/ModularInverse.mjs
new file mode 100644
index 00000000..88f4bff7
--- /dev/null
+++ b/src/core/operations/ModularInverse.mjs
@@ -0,0 +1,107 @@
+/**
+ * @author p-leriche [philip.leriche@cantab.net]
+ * @copyright Crown Copyright 2025
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import OperationError from "../errors/OperationError.mjs";
+import { parseBigInt, egcd } from "../lib/BigIntUtils.mjs";
+
+
+/* ---------- operation class ---------- */
+
+/**
+ * Modular Inverse operation
+ */
+class ModularInverse extends Operation {
+
+    /**
+     * ModularInverse constructor
+     */
+    constructor() {
+        super();
+
+        this.name = "Modular Inverse";
+        this.module = "Crypto";
+        this.description =
+            "Computes the modular multiplicative inverse of <i>a</i> modulo <i>m</i>.<br><br>" +
+            "Finds <i>x</i> such that a*x = 1 (mod m).<br><br>" +
+            "<b>Input handling:</b> If either <i>a</i> or <i>m</i> is left blank, " +
+            "its value is taken from the Input field.";
+        this.infoURL = "https://wikipedia.org/wiki/Modular_multiplicative_inverse";
+        this.inputType = "string";
+        this.outputType = "string";
+        this.args = [
+            {
+                name: "Value (a)",
+                type: "string",
+                value: ""
+            },
+            {
+                name: "Modulus (m)",
+                type: "string",
+                value: ""
+            }
+        ];
+    }
+
+    /**
+     * @param {string} input
+     * @param {Object[]} args
+     * @returns {string}
+     */
+    run(input, args) {
+        const [aStr, mStr] = args;
+
+        // Trim everything so "" and "   " count as empty
+        const aParam = aStr?.trim();
+        const mParam = mStr?.trim();
+        const inputVal = input?.trim();
+
+        let a, m;
+
+        if (aParam && mParam) {
+            // Case 1: value and modulus both given as parameters
+            a = aParam;
+            m = mParam;
+        } else if (!aParam && mParam) {
+            // Case 2: value missing - take from input
+            a = inputVal;
+            m = mParam;
+            if (!a) throw new OperationError("Value (a) must be defined");
+        } else if (aParam && !mParam) {
+            // Case 3: modulus missing - take from input
+            a = aParam;
+            m = inputVal;
+            if (!m) throw new OperationError("Modulus (m) must be defined");
+        } else if (!aParam && !mParam) {
+            // Case 4: value and modulus both missing
+            throw new OperationError("Value (a) and Modulus (m) must be defined");
+        }
+
+        const aBI = parseBigInt(a, "Value (a)");
+        const mBI = parseBigInt(m, "Modulus (m)");
+
+        if (mBI <= 0n) {
+            throw new OperationError("Modulus must be greater than zero");
+        }
+
+        const aNorm = ((aBI % mBI) + mBI) % mBI;
+        const [g, x] = egcd(aNorm, mBI);
+
+        if (g !== 1n && g !== -1n) {
+            throw new OperationError("Inverse does not exist because gcd(a, m) ≠ 1");
+        }
+
+        let inv = x;
+        if (g === -1n) inv = -inv;
+
+        inv = ((inv % mBI) + mBI) % mBI;
+
+
+        return inv.toString();
+    }
+}
+
+export default ModularInverse;
diff --git a/tests/operations/tests/ModularInverse.mjs b/tests/operations/tests/ModularInverse.mjs
new file mode 100644
index 00000000..53cb46a0
--- /dev/null
+++ b/tests/operations/tests/ModularInverse.mjs
@@ -0,0 +1,78 @@
+/**
+ * Modular Inverse tests.
+ *
+ * @author p-leriche [philip.leriche@cantab.net]
+ *
+ * @copyright Crown Copyright 2025
+ * @license Apache-2.0
+ */
+import TestRegister from "../../lib/TestRegister.mjs";
+
+TestRegister.addTests([
+    {
+        name: "Modular Inverse: basic example (3 mod 11)",
+        input: "",
+        expectedOutput: "4",
+        recipeConfig: [
+            {
+                op: "Modular Inverse",
+                args: ["3", "11"],
+            },
+        ],
+    },
+    {
+        name: "Modular Inverse: another coprime pair (7 mod 26)",
+        input: "",
+        expectedOutput: "15",
+        recipeConfig: [
+            {
+                op: "Modular Inverse",
+                args: ["7", "26"],
+            },
+        ],
+    },
+    {
+        name: "Modular Inverse: hexadecimal input (0x10 mod 0x11)",
+        input: "",
+        expectedOutput: "16",
+        recipeConfig: [
+            {
+                op: "Modular Inverse",
+                args: ["0x10", "0x11"],
+            },
+        ],
+    },
+    {
+        name: "Modular Inverse: using input field for value",
+        input: "5",
+        expectedOutput: "21",
+        recipeConfig: [
+            {
+                op: "Modular Inverse",
+                args: ["", "26"],
+            },
+        ],
+    },
+    {
+        name: "Modular Inverse: using input field for modulus",
+        input: "17",
+        expectedOutput: "7",
+        recipeConfig: [
+            {
+                op: "Modular Inverse",
+                args: ["5", ""],
+            },
+        ],
+    },
+    {
+        name: "Modular Inverse: large number (RSA-like)",
+        input: "",
+        expectedOutput: "934281398294",
+        recipeConfig: [
+            {
+                op: "Modular Inverse",
+                args: ["65537", "9999999999999"],
+            },
+        ],
+    },
+]);