The by-poll was necessitated as Munda has to enter the state Assembly within the mandatory six months period after becoming the chief minister on Sep 11 last year.
BJP sitting MLA Mangal Singh Soy had resigned two months ago to pave way for Munda, who will have to be elected to the state Assembly by Mar 11 to continue as CM.
Munda defeated Gagrai by over 17,000 votes, Returning Officer CK Singh said.
Besides Gagrai, five others, including Bahadur Oraon of CPI (ML) and four independents, were also in the fray. The polling took place on Feb 10.
Munda, a third time CM since 2003, will have to vacate his Jamshedpur Lok Sabha constituency, which he had won in the 2009 general elections on a BJP ticket.