A member of the Cook Islands Democratic Party, the MP for Avatiu, Albert Nicholas, has swapped allegiance and joined the governing Cook Islands Party.
The Cook Islands News reports the government says this will provide political certainty.
The country has been in political turmoil since last year's election with a series of electoral disputes decided in the courts.
This has meant the Cook Islands Party has been a minority government for months.
But Mr Nicholas' move will shore up their numbers, taking them to 12 in the 24 seat parliament, with one seat to be decided in a by-election at the end of this month.
Mr Nicholas has been given a cabinet portfolio.