Woolsey, the former leader of the Congressional Progressive Caucus, had been expected for months to make an announcement about her future plans by the end of June, and all indications since have been that she was leaning toward retiring.
She told the Press Democrat in December that she was considering retirement and wanted it known early so that potential successors could take steps toward running.
State Assemblyman Jared Huffman (D) had an exploratory committee open by January, and he had raised $120,000 for a bid by the end of the first quarter.
North Bay political activist Norman Solomon (D) filed candidacy papers with the Federal Election Commission in April, saying he would run for the seat if Woolsey retires.
Woolsey's 6th district includes Marin and Sonoma counties, and its southern border is the northern end of the Golden Gate Bridge. In the first redistricting plan under consideration in California, the district would extend north all the way to the Oregon border, taking all of Rep. Mike Thompson's (D) coastal area.
President Barack Obama won the district with 76 percent of the vote in 2008, and it would not appear to be any more competitive under the proposed map.