A Nigantha, follower of Nātaputta. He once visited the Buddha at Nālandā, and there followed a discussion on actions and their efficiency. Dīghatapassī reported this discussion to Nātaputta and Upāli, who was listening, went, against the advice of Dīghatapassī, to the Buddha and challenged him to a discussion, at the end of which Upāli was converted (M.i.371ff).
The Commentary (MA.ii.594) says that Dīghatapassī was long-limbed, hence his name.