The 40th parallel north is a circle of latitude that is 40 degrees north of the Earth's equatorial plane. It crosses Europe, the Mediterranean Sea, Asia, the Pacific Ocean, North America, and the Atlantic Ocean.

At this latitude the sun is visible for 15 hours, 1 minute during the summer solstice and 9 hours, 20 minutes during the winter solstice. On June 21, the sun is at 73.83 degrees in the sky and at 26.17 on 21 December.^{[citation needed]}

Starting at the Prime Meridian and heading eastwards, the parallel 40° north passes through:

The parallel 40° north forms the boundary between the State of Kansas and State of Nebraska. On 30 May 1854, the Kansas–Nebraska Act created the Territory of Kansas and the Territory of Nebraska divided by the parallel 40° north. Both territories were required to determine for themselves whether to permit slavery. Open conflict between free-state and pro-slavery forces in the Kansas Territory led to the American Civil War.

The parallel 40° north formed the original northern boundary of the British Colony of Maryland. A subsequent royal grant gave the Colony of Pennsylvania land north of the 40th parallel but mistakenly assumed it would intersect the Twelve Mile Circle, which it does not. Pennsylvania's border was thus unclear and the colony pushed for a border far south of the 40th parallel. The Mason–Dixon Line was drawn between 1763 and 1767 as the compromise boundary between the overlapping claims of these two colonies.

40th parallel may refer to: