There are a large number of outdoor basketball courts scattered throughout Seattle, with a smaller quantity of indoor courts. Seattle Parks and Recreations have installed basketball courts at a large number of parks throughout the city, and many community centers run by Seattle Parks have indoor basketball courts as well. You will also find outdoor courts at various Seattle public and private schools, and while these schools also may have indoor courts, these are generally not available for public use.
Seattle Parks and Recreations have close to 100 basketball courts in various parks spread throughout Seattle. Some of the smaller parks feature half courts, while the large parks typically have one or more full courts. The quality of the courts, hoops and backboards vary, but in general, they are typically in pretty good shape. At some of the larger parks with full courts, you may be able to find pick-up games in the early evenings and on weekends.
St Alphonsus School, in the neighborhood of Ballard, has four hoops on two full sized courts. As of January 2022, one of the hoops is broken and unusable. While the basketball courts are on private property (St. Alphonsus School is a private school), the courts are available to the general public outside of school hours and special events - a sign on the fence says to stay out of the area during school hours. There is ample street parking around the courts, and no available public restrooms.
St Alphonsus School Basketball Courts