First Avenue South Bridge, Pair of bascule bridges on State Route 99 over the Duwamish River in Seattle, US

The First Avenue South Bridge is a pair of double-leaf bascule bridges carrying State Route 99 over the Duwamish River, about three miles south of downtown Seattle. The northbound span dates from 1956; its approaches were demolished and rebuilt between 1996 and 1998, and the southbound span opened in February 1997.

The original northbound span opened in 1956 and linked industrial areas with residential neighborhoods to the south and southwest. Between 1996 and 1998 a major overhaul modernized the lifting mechanism and completely rebuilt the approach roads.

The bridge is named for First Avenue South, the road it carries, and its two spans are known locally as the First Avenue South Bridge pair. From its 1956 opening until February 1995, when the retrofit began, it recorded the highest motor vehicle accident rate in Washington state, a fact tied to its busy industrial corridor.

Vessels can pass through when the bridge opens, a process lasting about four minutes supervised by the US Coast Guard 13th District. The openings follow federal navigation rules, so travelers may occasionally experience traffic pauses.

From its opening in 1956 until February 1995, when the retrofit began, the First Avenue South Bridge had the highest motor vehicle accident rate in Washington state. The 2001 Nisqually earthquake knocked the southbound span's piers 3 inches out of alignment, closing it briefly for repairs.