I think it was TMA (trimethoxyamphetamine), although that had been synthesized before. Do you mean what was the first brand-new one?
Checking PIHKAL that is followed by MMDA (the 5-desoxy variant of 5-methyl-MDA you could say), which may actually be the first novel psychedelic... I cannot really find a chronological guideline to be more sure so this could all be wrong.