No, it’s not actually related to wheat, but that doesn’t mean it’s Paleo. Buckwheat is technically a pseudocereal that is commonly used as a grain replacement. It is still not welcome in a Paleo lifestyle though, because although it’s not quite as starchy as grains, buckwheat still ranks fairly high on the glycemic index and is only moderately nutritious. As with most grains and cereals, you’re far better off eating meat and vegetables when it comes to nutrition.
Gluten-Free ≠ Paleo
Many people use buckwheat as a gluten-free grain alternative, but just because a food is gluten-free, does not make it healthy. Another major reason we avoid grains is because of the amount of processing required to make them edible. By the time they’re done roasting, rinsing, and grinding these little seeds, they’re hardly even recognizable from their original form. That’s a pretty good basis on which to judge the suitability of your food.
Mark Sisson discusses the Primal suitability of Buckwheat in this post.