Ethier's blast capped a six-run uprising during which the Dodgers sent nine men to the plate. Tony Gwynn Jr. and Dee Gordon added run-scoring singles in the decisive frame.
Chad Billingsley (4-4) limited his opponents to one run over seven innings in his second consecutive start, limiting the Mariners to two hits while striking out eight. The right-hander issued three walks.
The National League West-leading Dodgers took two of their three games against Seattle.
Mariners starter Blake Beavan (3-6) lasted only two full innings after being roughed up for six runs on five hits.
Kyle Seager homered for Seattle.