South Korea's president has joined the pro-government Uri Party, after having been impeached for speaking in support of it ahead of elections last year.
Roh says he will not deal with day-to-day party matters
Roh Moo-hyun was accused of infringing an election law requiring public officials to maintain neutrality.
In general elections in April, the previously minor Uri Party achieved a majority in the National Assembly.
Last Friday, a Constitutional Court ruled that the president should be reinstated.
A spokesman for Mr Roh said he would not involve himself in day-to-day party matters, but would focus on a long-term reform agenda.
The party is made up of reformist legislators loyal to Mr Roh, and is a breakaway group of the former ruling Millennium Democratic Party (MDP), to which the president had belonged before resigning last September.
The MDP joined the largest opposition party, the Grand National Party (GNP), in voting to impeach the president, a former lawyer, in March.
However, the Constitutional Court ruled that, while Mr Roh had violated his neutrality as president by backing the liberal Uri party which was formed last October, this was insufficient to remove him from office.
It rejected the additional charges of corruption and incompetence.
Mr Roh's mandate was in effect renewed with a big vote of confidence in last month's parliamentary election.
The decision was a defeat for conservative politicians, who took a huge gamble when they voted to strip him of his powers.
Mr Roh apologised last week for the corruption allegations against his aides, and the party's involvement in fundraising abuses during the 2002 election campaign.
But he stopped short of apologising for electoral law violations which led to his impeachment.