Council members voted unanimously to approve the ordinance, which calls on businesses to start paying their employees at least $15 per hour by 2021.
Larger businesses will be expected to reach the $15 mark in three years, or four if they offer health insurance, according to the bill passed Monday. Small business have seven years.
A spokesman for Seattle Mayor Ed Murray said the mayor plans to sign the ordinance into law on Tuesday.
The International Franchise Organization has already announced plans to sue over a portion of the law grouping franchises with big businesses.
"The City Council's action today is unfair, discriminatory and a deliberate attempt to achieve a political agenda at the expense of small franchise businesses owners," IFA President and CEO Steve Caldeira said in a statement.
Workers in Washington state are already entitled to the nation's highest, state-level minimum wage -- $9.32 per hour. The federal minimum wage is $7.25.