Edward Lowassa, who was a leading opposition candidate in the 2015 presidential election, has defected to the governing party, Chama ChaMapinduzi (CCM)
Rtd Hon Edward Lowasa had been seen meeting President John Pombe Magufuli in state house on several occasions fuelling speculation that he would defect to CCM.
There has been a wave of defections by opposition MPs to the governing party, moves that are said to have weakened an already fragile opposition in Tanzania.
President Magufuli has been accused by rights groups and foreign governments of intimidating opposition parties.There has also been concerns about the freedom of expression.
Mr Lowassa had earlier served as prime minister from 2005 to 2008.