The 29-year-old joined Swansea in 2011 after leaving Newcastle United and had just under three years remaining on his previous deal.
Routledge's contract was set to expire at the end of June 2017, but the Welsh outfit have rewarded him with an extension after an impressive start to the season.
The former England Under-21 international provided an assist for Gylfi Sigurdsson in a victory against Manchester United in their first top-flight clash of the campaign, then saw his strike against West Brom nominated for goal of the month in August.
Manager Garry Monk told Swansea's official website: "He deserves it because in my time here he's probably been our most consistent performer – and he's still performing.
"His time before coming here was well documented but he's now found a home, somewhere he can be himself."
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