Pioneer Square is a neighborhood in the southwest corner of downtown Seattle, Washington, US. It was once the heart of the city: Seattle's founders settled there in 1852, following a brief six-month settlement at Alki Point on the far side of Elliott Bay. The early structures in the neighborhood were mostly wooden, and nearly all burned in the Great Seattle Fire of 1889. By the end of 1890, dozens of brick and stone buildings had been erected in their stead; to this day, the architectural character of the neighborhood derives from these late 19th century buildings, mostly examples of Richardsonian Romanesque.

The neighborhood takes its name from a small triangular plaza near the corner of First Avenue and Yesler Way, originally known as Pioneer Place. The Pioneer Square–Skid Road Historic District, a historic district including that plaza and several surrounding blocks, is listed on the National Register of Historic Places.

Like virtually all Seattle neighborhoods, the Pioneer Square neighborhood lacks definitive borders. It is bounded roughly by Alaskan Way S. on the west, beyond which are the docks of Elliott Bay; by S. King Street on the south, beyond which is SoDo; by 5th Avenue S. on the east, beyond which is the International District; and it extends between one and two blocks north of Yesler Way, beyond which is the rest of Downtown. Because Yesler Way marks the boundary between two different plats, the street grid north of Yesler does not line up with the neighborhood's other streets (nor with the compass), so the northern border of the district zigzags along numerous streets.

In some places, the Pioneer Square–Skid Road Historic District extends beyond these borders. It includes Union Station east of 4th Avenue S., and several city blocks south of S. King Street.

The settlement's importance was guaranteed in 1852, when Henry Yesler chose the site for his lumber mill, which was located on Elliott Bay at the foot of what is now Yesler Way, right on the border between the land claimed (and soon thereafter platted) by David Swinson "Doc" Maynard (to the south) and that platted by Arthur Denny and Carson Boren.

Much of the neighborhood is on landfill: in pioneer times, the area roughly between First and Second Avenue, bounded on the south by Jackson Street, and extending north almost to Yesler Way (about two-and-a-half city blocks) was a low-lying offshore island. The mainland shore roughly followed what is now Yesler Way to about Fourth Avenue, then ran southeast, at an angle of about 45 degrees to the current shoreline. Slightly inland were steep bluffs, which were largely smoothed away by regrading in the late 19th and early 20th century.

Yesler Way, originally Mill Street, is the main east–west street through the Pioneer Square neighborhood. Immediately south of the square itself, it was the dividing line between Maynard's original claim (to the south) and Boren's (to the north). It became Deadline, the northern border of the Great Restricted District, Maynardtown, Down on the Sawdust, the Lava Beds, the Tenderloin, White Chapel, or Wappyville, (after Charles Wappenstein, after a particularly corrupt police chief.), where low entertainment and vice were long tolerated. One of the earliest names, and one that stuck well into second half of the 20th century, was "Skid Row".

Henry Broderick, approaching his 80th birthday in 1959, wrote of the neighborhood south of Yesler, "[P]erhaps never in all history, certainly not in America, has there ever existed such a massive collection of the demimonde grouped in a restricted area." There were "parlor houses" with marquees, celebrity madames—among them Lou Graham, Lila Young, and Raw McRoberts—and piano "professors". Scrupulous in their dealings, the parlor houses were completely tolerated by the city at the time, but there were also the far more controversial "crib houses" such as the Midway, the Paris and Dreamland near the corner of Sixth Avenue South and King Street. Each had a hundred or more cubicles—"cribs"—and they were not known for any particular honesty in their dealings. The city health department conducted inspections and attempted to keep venereal disease under control, but the state of medicine at the time was not such as to give them any great chance of success. Besides the brothels there were "an ungodly mixture of dives, dumps ... pawnshops, hash houses, dope parlors and ... the et cetera that kept the police guessing." Gambling prospered, as did the Box houses: part theater, part bar, part brothel. Police only dared enter the neighborhood in teams. Perhaps the only safe haven in the neighborhood was the saloon "Our House", which rented out safe deposit boxes.