diff --git a/src/Semigroup.test.ts b/src/Semigroup.test.ts
new file mode 100644
index 0000000..9b754ec
--- /dev/null
+++ b/src/Semigroup.test.ts
@@ -0,0 +1,65 @@
+import { describe, expect, test } from "bun:test"
+
+import { Product, Str, Sum } from "./Semigroup.ts"
+
+describe("Semigroup", () => {
+  describe("Sum", () => {
+    test("should add values via concat", () => {
+      const a = new Sum(3)
+      const b = new Sum(5)
+      expect(a.concat(b).value).toBe(8)
+    })
+
+    test("should satisfy associativity law", () => {
+      const a = new Sum(1)
+      const b = new Sum(2)
+      const c = new Sum(3)
+      expect(a.concat(b).concat(c).value).toBe(a.concat(b.concat(c)).value)
+    })
+
+    test("should correctly render when apply toString()", () => {
+      const a = new Sum(42)
+      expect(a.toString()).toBe("Sum(42)")
+    })
+  })
+
+  describe("Product", () => {
+    test("should multiply values via concat", () => {
+      const a = new Product(3)
+      const b = new Product(5)
+      expect(a.concat(b).value).toBe(15)
+    })
+
+    test("should satisfy associativity law", () => {
+      const a = new Product(2)
+      const b = new Product(3)
+      const c = new Product(4)
+      expect(a.concat(b).concat(c).value).toBe(a.concat(b.concat(c)).value)
+    })
+
+    test("should correctly render when apply toString()", () => {
+      const a = new Product(7)
+      expect(a.toString()).toBe("Product(7)")
+    })
+  })
+
+  describe("Str", () => {
+    test("should concatenate strings via concat", () => {
+      const a = new Str("hello")
+      const b = new Str(" world")
+      expect(a.concat(b).value).toBe("hello world")
+    })
+
+    test("should satisfy associativity law", () => {
+      const a = new Str("foo")
+      const b = new Str("bar")
+      const c = new Str("baz")
+      expect(a.concat(b).concat(c).value).toBe(a.concat(b.concat(c)).value)
+    })
+
+    test("should correctly render when apply toString()", () => {
+      const a = new Str("hello")
+      expect(a.toString()).toBe("Str(hello)")
+    })
+  })
+})
diff --git a/src/Semigroup.ts b/src/Semigroup.ts
new file mode 100644
index 0000000..e56c2e6
--- /dev/null
+++ b/src/Semigroup.ts
@@ -0,0 +1,79 @@
+/**
+ * A Semigroup is a type with an associative binary operation (`concat`).
+ * For any values a, b, c: a.concat(b).concat(c) === a.concat(b.concat(c))
+ */
+export interface Semigroup<T> {
+  concat(other: T): T
+}
+
+/**
+ * A Semigroup under addition.
+ * Wraps a number and combines by adding values together.
+ */
+export class Sum implements Semigroup<Sum> {
+  readonly #value: number
+
+  constructor(value: number) {
+    this.#value = value
+  }
+
+  get value(): number {
+    return this.#value
+  }
+
+  concat(other: Sum): Sum {
+    return new Sum(this.#value + other.value)
+  }
+
+  toString(): string {
+    return `Sum(${this.#value})`
+  }
+}
+
+/**
+ * A Semigroup under multiplication.
+ * Wraps a number and combines by multiplying values together.
+ */
+export class Product implements Semigroup<Product> {
+  readonly #value: number
+
+  constructor(value: number) {
+    this.#value = value
+  }
+
+  get value(): number {
+    return this.#value
+  }
+
+  concat(other: Product): Product {
+    return new Product(this.#value * other.value)
+  }
+
+  toString(): string {
+    return `Product(${this.#value})`
+  }
+}
+
+/**
+ * A Semigroup under string concatenation.
+ * Wraps a string and combines by concatenating values together.
+ */
+export class Str implements Semigroup<Str> {
+  readonly #value: string
+
+  constructor(value: string) {
+    this.#value = value
+  }
+
+  get value(): string {
+    return this.#value
+  }
+
+  concat(other: Str): Str {
+    return new Str(this.#value + other.value)
+  }
+
+  toString(): string {
+    return `Str(${this.#value})`
+  }
+}
diff --git a/src/index.ts b/src/index.ts
index e414b20..f710c78 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -9,3 +9,4 @@ export * from "./Monad.ts"
 export * from "./Option.ts"
 export * from "./Pipe.ts"
 export * from "./Result.ts"
+export * from "./Semigroup.ts"