diff --git a/native/expo-modules/thumbhash/ios/Thumbhash.swift b/native/expo-modules/thumbhash/ios/Thumbhash.swift
new file mode 100644
--- /dev/null
+++ b/native/expo-modules/thumbhash/ios/Thumbhash.swift
@@ -0,0 +1,294 @@
+import Foundation
+import UIKit
+
+// Thumbhash implementation thanks to @evanw work
+// https://github.com/evanw/thumbhash
+// NOTE: Swift has an exponential-time type checker and compiling very simple
+// expressions can easily take many seconds, especially when expressions involve
+// numeric type constructors.
+//
+// This file deliberately breaks compound expressions up into separate variables
+// to improve compile time even though this comes at the expense of readability.
+// This is a known workaround for this deficiency in the Swift compiler.
+//
+// The following command is helpful when debugging Swift compile time issues:
+//
+//     swiftc ThumbHash.swift -Xfrontend -debug-time-function-bodies
+//
+// These optimizations brought the compile time for this file from around 2.5
+// seconds to around 250ms (10x faster).
+// NOTE: Swift's debug-build performance of for-in loops over numeric ranges is
+// really awful. Debug builds compile a very generic indexing iterator thing
+// that makes many nested calls for every iteration, which makes debug-build
+// performance crawl.
+//
+// This file deliberately avoids for-in loops that loop for more than a few
+// times to improve debug-build run time even though this comes at the expense
+// of readability. Similarly unsafe pointers are used instead of array getters
+// to avoid unnecessary bounds checks, which have extra overhead in debug builds
+//
+// These optimizations brought the run time to encode and decode 10 ThumbHashes
+// in debug mode from 700ms to 70ms (10x faster).
+// swiftlint:disable all
+private func rgbaToThumbHash(w: Int, h: Int, rgba: Data) -> Data {
+  // Encoding an image larger than 100x100 is slow with no benefit
+  assert(w <= 100 && h <= 100)
+  assert(rgba.count == w * h * 4)
+
+  // Determine the average color
+  var avg_r: Float32 = 0
+  var avg_g: Float32 = 0
+  var avg_b: Float32 = 0
+  var avg_a: Float32 = 0
+  rgba.withUnsafeBytes { rgba in
+    var rgba = rgba.baseAddress!.bindMemory(to: UInt8.self,
+                                            capacity: rgba.count)
+    let n = w * h
+    var i = 0
+    while i < n {
+      let alpha = Float32(rgba[3]) / 255
+      avg_r += alpha / 255 * Float32(rgba[0])
+      avg_g += alpha / 255 * Float32(rgba[1])
+      avg_b += alpha / 255 * Float32(rgba[2])
+      avg_a += alpha
+      rgba = rgba.advanced(by: 4)
+      i += 1
+    }
+  }
+  if avg_a > 0 {
+    avg_r /= avg_a
+    avg_g /= avg_a
+    avg_b /= avg_a
+  }
+
+  let hasAlpha = avg_a < Float32(w * h)
+  let l_limit = hasAlpha ? 5 : 7 // Use fewer luminance bits if there's alpha
+  let imax_wh = max(w, h)
+  let iwl_limit = l_limit * w
+  let ihl_limit = l_limit * h
+  let fmax_wh = Float32(imax_wh)
+  let fwl_limit = Float32(iwl_limit)
+  let fhl_limit = Float32(ihl_limit)
+  let flx = round(fwl_limit / fmax_wh)
+  let fly = round(fhl_limit / fmax_wh)
+  var lx = Int(flx)
+  var ly = Int(fly)
+  lx = max(1, lx)
+  ly = max(1, ly)
+  var lpqa = [Float32](repeating: 0, count: w * h * 4)
+
+  // Convert the image from RGBA to LPQA (composite atop the average color)
+  rgba.withUnsafeBytes { rgba in
+    lpqa.withUnsafeMutableBytes { lpqa in
+      var rgba = rgba.baseAddress!.bindMemory(to: UInt8.self,
+                                              capacity: rgba.count)
+      var lpqa = lpqa.baseAddress!.bindMemory(to: Float32.self,
+                                              capacity: lpqa.count)
+      let n = w * h
+      var i = 0
+      while i < n {
+        let alpha = Float32(rgba[3]) / 255
+        let r = avg_r * (1 - alpha) + alpha / 255 * Float32(rgba[0])
+        let g = avg_g * (1 - alpha) + alpha / 255 * Float32(rgba[1])
+        let b = avg_b * (1 - alpha) + alpha / 255 * Float32(rgba[2])
+        lpqa[0] = (r + g + b) / 3
+        lpqa[1] = (r + g) / 2 - b
+        lpqa[2] = r - g
+        lpqa[3] = alpha
+        rgba = rgba.advanced(by: 4)
+        lpqa = lpqa.advanced(by: 4)
+        i += 1
+      }
+    }
+  }
+
+  // Encode using the DCT into DC (constant) and normalized AC (varying) terms
+  let encodeChannel = { (channel: UnsafePointer<Float32>, nx: Int, ny: Int) ->
+    (Float32, [Float32], Float32) in
+    var dc: Float32 = 0
+    var ac: [Float32] = []
+    var scale: Float32 = 0
+    var fx = [Float32](repeating: 0, count: w)
+    fx.withUnsafeMutableBytes { fx in
+      let fx = fx.baseAddress!.bindMemory(to: Float32.self, capacity: fx.count)
+      var cy = 0
+      while cy < ny {
+        var cx = 0
+        while cx * ny < nx * (ny - cy) {
+          var ptr = channel
+          var f: Float32 = 0
+          var x = 0
+          while x < w {
+            let fw = Float32(w)
+            let fxx = Float32(x)
+            let fcx = Float32(cx)
+            fx[x] = cos(Float32.pi / fw * fcx * (fxx + 0.5))
+            x += 1
+          }
+          var y = 0
+          while y < h {
+            let fh = Float32(h)
+            let fyy = Float32(y)
+            let fcy = Float32(cy)
+            let fy = cos(Float32.pi / fh * fcy * (fyy + 0.5))
+            var x = 0
+            while x < w {
+              f += ptr.pointee * fx[x] * fy
+              x += 1
+              ptr = ptr.advanced(by: 4)
+            }
+            y += 1
+          }
+          f /= Float32(w * h)
+          if cx > 0 || cy > 0 {
+            ac.append(f)
+            scale = max(scale, abs(f))
+          } else {
+            dc = f
+          }
+          cx += 1
+        }
+        cy += 1
+      }
+    }
+    if scale > 0 {
+      let n = ac.count
+      var i = 0
+      while i < n {
+        ac[i] = 0.5 + 0.5 / scale * ac[i]
+        i += 1
+      }
+    }
+    return (dc, ac, scale)
+  }
+  let (
+    (l_dc, l_ac, l_scale),
+    (p_dc, p_ac, p_scale),
+    (q_dc, q_ac, q_scale),
+    (a_dc, a_ac, a_scale)
+  ) = lpqa.withUnsafeBytes { lpqa in
+    let lpqa = lpqa.baseAddress!.bindMemory(to: Float32.self,
+                                            capacity: lpqa.count)
+    return (
+      encodeChannel(lpqa, max(3, lx), max(3, ly)),
+      encodeChannel(lpqa.advanced(by: 1), 3, 3),
+      encodeChannel(lpqa.advanced(by: 2), 3, 3),
+      hasAlpha ? encodeChannel(lpqa.advanced(by: 3), 5, 5) : (1, [], 1)
+    )
+  }
+
+  // Write the constants
+  let isLandscape = w > h
+  let fl_dc = round(63.0 * l_dc)
+  let fp_dc = round(31.5 + 31.5 * p_dc)
+  let fq_dc = round(31.5 + 31.5 * q_dc)
+  let fl_scale = round(31.0 * l_scale)
+  let il_dc = UInt32(fl_dc)
+  let ip_dc = UInt32(fp_dc)
+  let iq_dc = UInt32(fq_dc)
+  let il_scale = UInt32(fl_scale)
+  let ihasAlpha = UInt32(hasAlpha ? 1 : 0)
+  let header24 = il_dc
+    | (ip_dc << 6)
+    | (iq_dc << 12)
+    | (il_scale << 18)
+    | (ihasAlpha << 23)
+  let fp_scale = round(63.0 * p_scale)
+  let fq_scale = round(63.0 * q_scale)
+  let ilxy = UInt16(isLandscape ? ly : lx)
+  let ip_scale = UInt16(fp_scale)
+  let iq_scale = UInt16(fq_scale)
+  let iisLandscape = UInt16(isLandscape ? 1 : 0)
+  let header16 = ilxy | (ip_scale << 3) | (iq_scale << 9) | (iisLandscape << 15)
+  var hash = Data(capacity: 25)
+  hash.append(UInt8(header24 & 255))
+  hash.append(UInt8((header24 >> 8) & 255))
+  hash.append(UInt8(header24 >> 16))
+  hash.append(UInt8(header16 & 255))
+  hash.append(UInt8(header16 >> 8))
+  var isOdd = false
+  if hasAlpha {
+    let fa_dc = round(15.0 * a_dc)
+    let fa_scale = round(15.0 * a_scale)
+    let ia_dc = UInt8(fa_dc)
+    let ia_scale = UInt8(fa_scale)
+    hash.append(ia_dc | (ia_scale << 4))
+  }
+
+  // Write the varying factors
+  for ac in [l_ac, p_ac, q_ac] {
+    for f in ac {
+      let f15 = round(15.0 * f)
+      let i15 = UInt8(f15)
+      if isOdd {
+        hash[hash.count - 1] |= i15 << 4
+      } else {
+        hash.append(i15)
+      }
+      isOdd = !isOdd
+    }
+  }
+  if hasAlpha {
+    for f in a_ac {
+      let f15 = round(15.0 * f)
+      let i15 = UInt8(f15)
+      if isOdd {
+        hash[hash.count - 1] |= i15 << 4
+      } else {
+        hash.append(i15)
+      }
+      isOdd = !isOdd
+    }
+  }
+  return hash
+}
+
+func thumbHash(fromImage: UIImage) -> Data {
+  let size = fromImage.size
+  let w = Int(round(100 * size.width / max(size.width, size.height)))
+  let h = Int(round(100 * size.height / max(size.width, size.height)))
+  var rgba = Data(count: w * h * 4)
+  rgba.withUnsafeMutableBytes { rgba in
+    if
+      let space = fromImage.cgImage?.colorSpace,
+      let context = CGContext(
+        data: rgba.baseAddress,
+        width: w,
+        height: h,
+        bitsPerComponent: 8,
+        bytesPerRow: w * 4,
+        space: space,
+        bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue
+      )
+    {
+      // EXIF orientation only works if you draw the UIImage, not the CGImage
+      context.concatenate(CGAffineTransform(1, 0, 0, -1, 0, CGFloat(h)))
+      UIGraphicsPushContext(context)
+      fromImage.draw(in: CGRect(x: 0, y: 0, width: w, height: h))
+      UIGraphicsPopContext()
+
+      // Convert from premultiplied alpha to unpremultiplied alpha
+      var rgba = rgba.baseAddress!.bindMemory(to: UInt8.self,
+                                              capacity: rgba.count)
+      let n = w * h
+      var i = 0
+      while i < n {
+        let a = UInt16(rgba[3])
+        if a > 0 && a < 255 {
+          var r = UInt16(rgba[0])
+          var g = UInt16(rgba[1])
+          var b = UInt16(rgba[2])
+          r = min(255, r * 255 / a)
+          g = min(255, g * 255 / a)
+          b = min(255, b * 255 / a)
+          rgba[0] = UInt8(r)
+          rgba[1] = UInt8(g)
+          rgba[2] = UInt8(b)
+        }
+        rgba = rgba.advanced(by: 4)
+        i += 1
+      }
+    }
+  }
+  return rgbaToThumbHash(w: w, h: h, rgba: rgba)
+}
diff --git a/native/expo-modules/thumbhash/ios/ThumbhashModule.swift b/native/expo-modules/thumbhash/ios/ThumbhashModule.swift
--- a/native/expo-modules/thumbhash/ios/ThumbhashModule.swift
+++ b/native/expo-modules/thumbhash/ios/ThumbhashModule.swift
@@ -4,8 +4,25 @@
   public func definition() -> ModuleDefinition {
     Name("Thumbhash")
 
-    AsyncFunction("generateThumbHash") { () -> String in
-      "unimplemented"
-    }
+    AsyncFunction("generateThumbHash", generateThumbHash)
+  }
+}
+
+// MARK: - Function implementations
+
+private func generateThumbHash(photoURI: URL) throws -> String {
+  let imageData = try Data(contentsOf: photoURI)
+  guard let image = UIImage(data: imageData) else {
+    throw LoadingFailureException(photoURI.absoluteString)
+  }
+
+  return thumbHash(fromImage: image).base64EncodedString()
+}
+
+// MARK: - Exception definitions
+
+private class LoadingFailureException: GenericException<String> {
+  override var reason: String {
+    "Failed to load image URI: \(param)"
   }
 }