Is the Gulf of Mexico the Largest Gulf in the World?

No. The Gulf of Mexico (~1.55 million km²) is the 9th largest gulf. The largest is the Gulf of Guinea (NE tropical Atlantic, West Africa) at ~2.35 million km².

Top Gulfs by Area

1. Gulf of Guinea — 2.35M km²
2. Gulf of Alaska — 1.53M km² (marginal sea)
3. Gulf of Mexico — 1.55M km²
4. Bay of Bengal — 2.17M km² (often classed as bay)
5. Hudson Bay — 1.23M km² (inland sea)
6. Persian Gulf — 251,000 km²

Definitions vary (gulf vs. bay vs. sea), but by standard hydrographic classification, the Gulf of Guinea is largest. The Gulf of Mexico is the largest gulf entirely within the Atlantic Ocean basin and bordering the US.