Rosana Tositrakul will officially announce her decision to contest the Bangkok gubernatorial election on Friday.
The long-time activist and former senator said in September that she would run for Bangkok governor as an independent candidate.
On Friday she will officially announce the details, according to a campaign poster distributed to the media on Monday.
No date has yet been set for city election day.
She will certainly face former transport minister Chadchart Sittipunt, who announced last month his bid for the top job in the capital, also as an independent candidate instead of under the Pheu Thai Party banner.
The gubernatorial poll in Bangkok has been on hold since then-army chief Gen Prayut Chan-o-cha seized power in the coup of May 2004. Junta appointees have filled the position.