The noodle paired with tempura in tempura soba is soba.
Soba are Japanese noodles made primarily from buckwheat flour, often with some wheat flour added for strength. They may be served chilled with a dipping sauce or hot in a broth. In tempura soba—also called tensoba—a piece of tempura, commonly shrimp or a mixed vegetable fritter, is served with the noodles.
The pairing works because soba’s earthy, slightly nutty flavor contrasts with the delicate fried coating. Depending on the restaurant or region, the tempura may sit in the broth, rest on top of the noodles, or be served separately to preserve its crispness. Vegetable tempura is also common.
Soba is easy to confuse with udon, ramen, and somen, but they differ in ingredients and texture. Soba is associated with buckwheat; udon is thick and wheat-based, ramen uses Chinese-style wheat noodles, and somen is typically very thin wheat noodle. Tempura udon is therefore a different dish.