When first posted as such in January 1964, SR 303 started at Jct SR 304 in Bremerton and headed north via the Manette Bridge to a point about a mile west of Keyport and then headed west to Jct SR 3, where SR 303 ended. A branch of SR 303 began at Jct SR 304 a mile or so west of the route earlier described in Bremerton, and headed north via the Warren Ave Bridge, joining the other route of SR 303 a mile or so north of the Warren Ave Bridge. Yet another branch started a mile or so west of Keyport and headed east to Keyport.
The E-W portion of SR 303 between Keyport and Jct SR 3 was renumbered SR 308 in 1971. Circa 1980, Kitsap County built an E-W freeway from Jct SR 3 in Silverdale to Jct SR 303 east of Silverdale. The county called this route Waaga Way. Effective April 1, 1992, SR 303 from Jct SR 304 via the Manette Bridge to the junction with the Warren Ave Bridge branch of SR 303 was dropped from the state highway system. The Manette Bridge itself remained a state responsibility. The route of SR 303 from Waaga Way north to Jct SR 308 was dropped from the state highway system while SR 303 was routed west via Waaga Way to Jct SR 3 at Silverdale. Today, SR 303 is 9.52 miles long.
Before 1964, SR 303 from Bremerton to the junction about a mile west of Keyport mentioned above was SSH 21B. The east-west portion of SR 303, now designated SR 308, had been a branch of PSH 21.
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