diff --git a/pyproject.toml b/pyproject.toml
index 299ea51..8fbf919 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -11,6 +11,7 @@ dependencies = [
     "opencv-python>=4.10",
     "Pillow>=11.0",
     "anthropic>=0.52",
+    "numpy>=1.26",
 ]
 
 [project.optional-dependencies]
diff --git a/tests/test_overlay.py b/tests/test_overlay.py
new file mode 100644
index 0000000..f9e8bd6
--- /dev/null
+++ b/tests/test_overlay.py
@@ -0,0 +1,41 @@
+from pathlib import Path
+
+import numpy as np
+from PIL import Image
+
+from unredact.pipeline.overlay import render_overlay
+from unredact.pipeline.ocr import ocr_page
+from unredact.pipeline.font_detect import detect_font
+from unredact.pipeline.rasterize import rasterize_pdf
+
+
+def test_overlay_same_size_as_original(sample_pdf: Path):
+    pages = rasterize_pdf(sample_pdf, first_page=1, last_page=1)
+    lines = ocr_page(pages[0])
+    font_match = detect_font(lines, pages[0])
+    result = render_overlay(pages[0], lines, font_match)
+    assert result.size == pages[0].size
+
+
+def test_overlay_has_green_pixels(sample_pdf: Path):
+    pages = rasterize_pdf(sample_pdf, first_page=1, last_page=1)
+    lines = ocr_page(pages[0])
+    font_match = detect_font(lines, pages[0])
+    result = render_overlay(pages[0], lines, font_match)
+    # Convert to numpy and check for green-ish pixels
+    arr = np.array(result)
+    # Green channel significantly higher than red and blue
+    green_mask = (arr[:, :, 1] > 100) & (arr[:, :, 0] < 100) & (arr[:, :, 2] < 100)
+    green_pixel_count = green_mask.sum()
+    assert green_pixel_count > 100, "Expected green text pixels in the overlay"
+
+
+def test_overlay_preserves_original(sample_pdf: Path):
+    """Overlay should be composited — original should still be visible."""
+    pages = rasterize_pdf(sample_pdf, first_page=1, last_page=1)
+    original = pages[0].copy()
+    lines = ocr_page(pages[0])
+    font_match = detect_font(lines, pages[0])
+    result = render_overlay(pages[0], lines, font_match)
+    # The result shouldn't be identical to the original (overlay adds green)
+    assert list(result.getdata()) != list(original.getdata())
diff --git a/unredact/pipeline/overlay.py b/unredact/pipeline/overlay.py
new file mode 100644
index 0000000..10371c0
--- /dev/null
+++ b/unredact/pipeline/overlay.py
@@ -0,0 +1,44 @@
+from PIL import Image, ImageDraw, ImageFont
+
+from unredact.pipeline.font_detect import FontMatch
+from unredact.pipeline.ocr import OcrLine
+
+
+def render_overlay(
+    page_image: Image.Image,
+    lines: list[OcrLine],
+    font_match: FontMatch,
+    color: tuple[int, int, int, int] = (0, 200, 0, 160),
+) -> Image.Image:
+    """Render green text overlay on top of the document image.
+
+    Draws each OCR'd line's text at its detected position using
+    the matched font. The overlay is semi-transparent so the
+    original document is still visible underneath.
+
+    Args:
+        page_image: The original rasterized page.
+        lines: OCR'd lines with character positions.
+        font_match: Detected font to use for rendering.
+        color: RGBA color for the overlay text.
+
+    Returns:
+        A new RGBA image with the overlay composited on the original.
+    """
+    base = page_image.convert("RGBA")
+    overlay = Image.new("RGBA", base.size, (0, 0, 0, 0))
+    draw = ImageDraw.Draw(overlay)
+    font = font_match.to_pil_font()
+
+    for line in lines:
+        if not line.chars:
+            continue
+
+        text = line.text
+        # Position: use the first character's x and the line's y
+        x = line.x
+        y = line.y
+
+        draw.text((x, y), text, font=font, fill=color)
+
+    return Image.alpha_composite(base, overlay)