From the Guidebook:
Named for the U.S.S. Portsmouth,
commanded by Captain John B. Montgomery, after whom Montgomery
Street was named. It was here on the plaza that Captain
Montgomery first raised the American flag near the Mexican
adobe custom house on July 9, 1846. Center of many early-day
activities, this plaza was the site of: the first public
school building, erected in 1847 on the southwest corner
of plaza, where religious services and many public meetings
were held; the dramatic announcement of gold discovery
made on May 11, 1848, when Sam Brannan displayed glittering
samples to crowds; the mass meeting to urge election of
delegates to Monterey Constitutional Convention on June
12, 1849; refuge for citizens following conflagrations
of 1849, 1850, 1851, and 1906; a citizens' assembly on
July 16, 1849 to organize against depredations of a lawless
body called the "Hounds;"' memorial services
held August 29, 1850, following death of President Zachary
Taylor; first Admission Day celebration held October 29,
1850, when the steamship Oregon brought the news that
California had become 31st state on September 9; an indignation
meeting, organized June 1, 1852, to protest against the
city council's purchase of the Jenny Lind Theatre to be
used as a city hall; commemorative services held for Henry
Daly, August 10, 1852; and an oration delivered by Colonel
E. D. Baker on September 18, 1859, over the body of U.
S. Senator David C. Broderick, killed in duel with Chief
Justice David S. Terry. Robert Louis Stevenson spent many
hours here during his visit to the city in 1879-1880.
From the plaque:
On this spot the American flag was first raised in San Francisco by Commander John B. Montgomery of the U.S.S. Portsmouth July 9, 1846.
Marked by the Daughters of the American Revolution, San Francisco Chapters, 1924.