diff --git a/src/core/config/Categories.json b/src/core/config/Categories.json
index 434c8bb619..61b6d64d27 100644
--- a/src/core/config/Categories.json
+++ b/src/core/config/Categories.json
@@ -33,6 +33,8 @@
             "Show Base64 offsets",
             "To Base92",
             "From Base92",
+            "To Base91",
+            "From Base91",
             "To Base85",
             "From Base85",
             "To Base",
diff --git a/src/core/lib/Base91.mjs b/src/core/lib/Base91.mjs
new file mode 100644
index 0000000000..da9e296f4f
--- /dev/null
+++ b/src/core/lib/Base91.mjs
@@ -0,0 +1,125 @@
+/**
+ * Base91 resources.
+ *
+ * Based on the original basE91 algorithm by Joachim Henke
+ * http://base91.sourceforge.net/
+ *
+ * @author CyberChef Base91 Implementation
+ * @copyright Crown Copyright 2024
+ * @license Apache-2.0
+ * @modified-by Izai Alejandro Zalles Merino <zallesrene@gmail.com> (ialejandrozalles)
+ * @modified-date 2025-10-01
+ * © 2025 Izai Alejandro Zalles Merino
+
+ */
+
+import OperationError from "../errors/OperationError.mjs";
+
+/**
+ * Base91 alphabet - 91 printable ASCII characters
+ */
+const BASE91_ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!#$%&()*+,./:;<=>?@[]^_`{|}~\"";
+
+/**
+ * Decode table for Base91
+ */
+const BASE91_DECODE_TABLE = new Array(256).fill(-1);
+for (let i = 0; i < BASE91_ALPHABET.length; i++) {
+    BASE91_DECODE_TABLE[BASE91_ALPHABET.charCodeAt(i)] = i;
+}
+
+/**
+ * Encode bytes to Base91
+ *
+ * @param {Uint8Array} data - Input byte array
+ * @returns {string} Base91 encoded string
+ * @modified-by Izai Alejandro Zalles Merino <zallesrene@gmail.com> (ialejandrozalles)
+
+ */
+export function encodeBase91(data) {
+    let accumulator = 0;
+    let accumulatorBits = 0;
+    let output = "";
+
+    for (let i = 0; i < data.length; i++) {
+        accumulator |= data[i] << accumulatorBits;
+        accumulatorBits += 8;
+
+        if (accumulatorBits > 13) {
+            let value = accumulator & 8191;
+
+            if (value > 88) {
+                accumulator >>= 13;
+                accumulatorBits -= 13;
+            } else {
+                value = accumulator & 16383;
+                accumulator >>= 14;
+                accumulatorBits -= 14;
+            }
+
+            output += BASE91_ALPHABET[value % 91] + BASE91_ALPHABET[Math.floor(value / 91)];
+        }
+    }
+
+    if (accumulatorBits > 0) {
+        output += BASE91_ALPHABET[accumulator % 91];
+
+        if (accumulatorBits > 7 || accumulator > 90) {
+            output += BASE91_ALPHABET[Math.floor(accumulator / 91)];
+        }
+    }
+
+    return output;
+}
+
+/**
+ * Decode Base91 string to bytes
+ *
+ * @param {string} str - Base91 encoded string
+ * @returns {Uint8Array} Decoded byte array
+ * @modified-by Izai Alejandro Zalles Merino <zallesrene@gmail.com> (ialejandrozalles)
+
+ */
+export function decodeBase91(str) {
+    let accumulator = 0;
+    let accumulatorBits = 0;
+    let value = -1;
+    const output = [];
+
+    for (let i = 0; i < str.length; i++) {
+        const charCode = str.charCodeAt(i);
+        const decodeValue = BASE91_DECODE_TABLE[charCode];
+
+        if (decodeValue === -1) {
+            throw new OperationError(`Invalid Base91 character: ${str[i]}`);
+        }
+
+        if (value === -1) {
+            value = decodeValue;
+        } else {
+            value += decodeValue * 91;
+            accumulator |= (value << accumulatorBits);
+
+            if (value > 88) {
+                accumulatorBits += 13;
+            } else {
+                accumulatorBits += 14;
+            }
+
+            value = -1;
+
+            while (accumulatorBits > 7) {
+                output.push(accumulator & 255);
+                accumulator >>= 8;
+                accumulatorBits -= 8;
+            }
+        }
+    }
+
+    if (value !== -1) {
+        accumulator |= value << accumulatorBits;
+        output.push(accumulator & 255);
+    }
+
+    return new Uint8Array(output);
+}
diff --git a/src/core/operations/FromBase91.mjs b/src/core/operations/FromBase91.mjs
new file mode 100644
index 0000000000..921434dd76
--- /dev/null
+++ b/src/core/operations/FromBase91.mjs
@@ -0,0 +1,39 @@
+/**
+ * @author Izai Alejandro Zalles Merino <zallesrene@gmail.com> (ialejandrozalles)
+ * @copyright © 2025 Izai Alejandro Zalles Merino
+ * @license Apache-2.0
+ */
+
+import { decodeBase91 } from "../lib/Base91.mjs";
+import Operation from "../Operation.mjs";
+
+/**
+ * From Base91 operation
+ */
+class FromBase91 extends Operation {
+    /**
+     * FromBase91 constructor
+     */
+    constructor() {
+        super();
+
+        this.name = "From Base91";
+        this.module = "Default";
+        this.description = "Base91 is a binary-to-text encoding scheme that uses 91 printable ASCII characters. It provides better space efficiency than Base64 while maintaining readability. This operation decodes Base91-encoded text back to its original binary data.";
+        this.infoURL = "https://en.wikipedia.org/wiki/Binary-to-text_encoding#Encoding_standards";
+        this.inputType = "string";
+        this.outputType = "ArrayBuffer";
+    }
+
+    /**
+     * @param {string} input
+     * @param {Object[]} args
+     * @returns {ArrayBuffer}
+     */
+    run(input, args) {
+        const decoded = decodeBase91(input);
+        return decoded.buffer.slice(decoded.byteOffset, decoded.byteOffset + decoded.byteLength);
+    }
+}
+
+export default FromBase91;
diff --git a/src/core/operations/ToBase91.mjs b/src/core/operations/ToBase91.mjs
new file mode 100644
index 0000000000..88fa5cfb5b
--- /dev/null
+++ b/src/core/operations/ToBase91.mjs
@@ -0,0 +1,39 @@
+/**
+ * @author Izai Alejandro Zalles Merino <zallesrene@gmail.com> (ialejandrozalles)
+ * @copyright © 2025 Izai Alejandro Zalles Merino
+ * @license Apache-2.0
+ */
+
+import { encodeBase91 } from "../lib/Base91.mjs";
+import Operation from "../Operation.mjs";
+
+/**
+ * To Base91 operation
+ */
+class ToBase91 extends Operation {
+    /**
+     * ToBase91 constructor
+     */
+    constructor() {
+        super();
+
+        this.name = "To Base91";
+        this.module = "Default";
+        this.description = "Base91 is a binary-to-text encoding scheme that uses 91 printable ASCII characters. It provides better space efficiency than Base64 while maintaining readability. Base91 encodes arbitrary binary data using characters A-Z, a-z, 0-9, and various symbols (excluding hyphen, backslash, and single quote).";
+        this.infoURL = "https://en.wikipedia.org/wiki/Binary-to-text_encoding#Encoding_standards";
+        this.inputType = "ArrayBuffer";
+        this.outputType = "string";
+    }
+
+    /**
+     * @param {ArrayBuffer} input
+     * @param {Object[]} args
+     * @returns {string}
+     */
+    run(input, args) {
+        const data = new Uint8Array(input);
+        return encodeBase91(data);
+    }
+}
+
+export default ToBase91;
diff --git a/tests/node/tests/nodeApi.mjs b/tests/node/tests/nodeApi.mjs
index 29a47ffc8d..2aa1c5fec0 100644
--- a/tests/node/tests/nodeApi.mjs
+++ b/tests/node/tests/nodeApi.mjs
@@ -136,7 +136,7 @@ TestRegister.addApiTests([
 
     it("chef.help: returns multiple results", () => {
         const result = chef.help("base 64");
-        assert.strictEqual(result.length, 13);
+        assert.strictEqual(result.length, 15);
     }),
 
     it("chef.help: looks in description for matches too", () => {