Outside of T-Mobile Park, the current name of the Mariner's stadium, and a couple of colleges, almost all Seattle baseball fields are part of the park system managed by the Seattle Parks and Recreations. While a handful of the Seattle Parks baseball fields are available for drop-in play, most require a reservation to use. See below for more information.
Seattle Parks and Recreation manage a number of playfields throughout the city of Seattle, most of which include one or more baseball fields. Thirty baseball fields have synthetic surfaces (entire field or just the outfield), which is nice during the rainy months, and you will find lights on some of the newer baseball fields. Most baseball fields require a paid reservation to use, and while not technically allowed, it's fairly common to see small groups using these fields when not in use. Seattle Parks has designated a small number of baseball fields as drop-in and fee free during certain hours, including Miller, Washington Park, Bobby Morris, Lower Woodland, Montlake, Brighton, Genesee Upper, Georgetown, Van Asselt Elementary, and Walt Hundley.