The bomb exploded at 7 pm (5:30 IST) outside Erawan Shrine in the downtown Chidlom district, police said.
The temple, which is dedicated to Lord Brahma, is located on a main road through Bangkok's commercial hub and is surrounded by three major shopping malls.
Thai TV said at least 15 people were killed and scores injured during the evening rush hour.
There was a huge amount of chaos, with body parts scattered everywhere, media reports said.
Police, emergency vans and bomb detonation squads rushed to the area where several motorcycles lay mangled.
Bomb attacks in Bangkok are extremely rare.