Dr Vincent Wijeysingha, who in July became the first politician to declare he is gay, has resigned from the Singapore Democratic Party.
In a media statement, the SDP said Dr Wijeysingha has expressed his desire to get more involved in lesbian, gay, bisexual and transgender (LGBT) and other human rights issues, and that "he finds it more effective to do it as an activist in the civil society arena".
The party added that while they were disappointed, they understood and fully support his position.
"Our loss is civil society's gain. We know that Dr Wijeysingha will carry on his work with honesty and courage. The Party has benefited immensely from his keen intellect, and we will miss his participation and contribution," it added.
In a Facebook post on Wednesday, Dr Wijeysingha, who contested in the last General Election for SDP in Holland-Bukit Timah GRC, said his recent post had attracted a debate on LGBT rights.
"The shape of the discussion shows there is misunderstanding of these issues, primarily because of lack of mainstream access to appropriate information," he wrote.
"Misunderstanding leads to discrimination that works its way into the lives of LGBT people, resulting in anguish and distress."
His last three years in the SDP, he said, were spent on in social and economic bread-and-butter policy issues.
But after reflection and consultation with friends and colleagues, Dr Wijeysingha said he believes he should participate in the "more intangible but no less important" work of promoting civil liberties. This is a project which he believes to be the dominant mission of this present period, he added.
"I continue to share deeply the values of the party which gave me a political home and the opportunity to learn an ideal of service which I have tried to carry out. I intend to explore ways in which I can honour those values in the civil liberties sphere."