McDermott, 79, said his decision not to see re-election and to retire was influenced by a few events in the past couple of years including the retirement of longtime congressional colleague and the death of a friend, he told a news conference in Seattle.
McDermott will retire by the end of this year after his term ends.
"I realised that life doesn't - you don't get forever," McDermott said during a reflective news conference in downtown Seattle.
"This was not an easy decision to make, because I don't like to quit. There are things I still want to do, see happen, but there's a time that comes and you say, enough," McDermott said.
More From This Section
In a statement, US President Barack Obama said for more than 40 years, McDermott has worked tirelessly on behalf of the people of Washington State.
"As a state legislator, he helped pass laws that offered healthcare to unemployed and low-income Washingtonians, the first such programme in the nation.
In the US Congress, he continued to be a much-needed voice for his most vulnerable constituents," Obama said.
Democratic leader Nancy Pelosi said McDermott has been a tenacious champion of hard-working Americans in Washington and across the US.
"Throughout all of his four decades of distinguished public service, McDermott has shown the strength of his progressive values and the quality of his leadership in expanding opportunity for all Americans," she said.
A former Co-Chair of the Congressional Caucus on India and Indian Americans, McDermott played a key role in a bringing India and US together and passage of the civil nuclear deal.
"The US and India were both British colonies, and both attained their independence through years of persistent struggle. The US and India are also two of the most ethnically, culturally and religiously diverse nations in the world," he added.