Undhad had left BJP just before the 2007 Assembly elections alleging that chief minister Narendra Modi was running the party in a dictatorial manner.
He joined BJP on Monday at its state headquarters here. He said: "The country is in need of leadership of Narendra Modi".
Undhad hails from numerically and politically strong Leuva Patel community.
The MLA said the country today needed a leader like Narendra Modi
The MLA said he was returning to BJP which gave him footing in politics. He contested on Congress ticket in 2007 and 2012 Assembly elections and emerged victorious.
"I have joined BJP today with my supporters and party workers. Seeing the current situation our country is in, it is important that Narendra Modi becomes Prime Minister of India", Undhad said at the party headquarters in Khanpur here.
State BJP president R C Faldu, outgoing Rajya Sabha MP Purshottam Rupala and former agriculture minister Dileep Sanghani were present on the occasion.
Earlier, in the day he submitted his resignation to Assembly speaker Vajubhi Vala, which was accepted.
On quitting Congress at this juncture, Undhad said, "I have joined BJP after holding several discussions and deliberations with my supporters and party leaders.However Congress did no injustice to me".
When asked how he could support Modi whom he revolted against before the 2007 polls, he said, "Seeing the current political situation in the country, it is important that Narendra Modi becomes prime minister. India requires the leadership of Narendra Modi. The Congress will be wiped out in the coming general elections".
"I acted as responsible opposition (Congress) leader while in the state Assembly. I was doing what my party was asking me to do", Undhad said.
On whether he would contest the upcoming Lok Sabha polls from Amreli constituency, he said, "the party would decide whether he should contest or not".
Undhad's resignation has brought down Congress' strength in the Assembly to 56 against BJP's 119 excluding him. His resignation has necessitated by-election.