Unknown American Indian women appeared on a Columbus Stamp of 1893 and an unknown pioneer woman can be seen on an 1898 issue. Martha Washington is the first actual woman to get her own stamp- in the regular issues series of 1902-1903.
Harriet Tubman
The first African-American woman honored on her own stamp was Harriet Tubman in 1978. Some of the letter-sorters shown on a 1973 stamp appear to black women.
The first American woman to be featured on a US stamp is Martha Washington. The stamp was released in America in 1902.
US Scott #306 Martha Washington was the first American woman to appear on a US Postage stamp on that date. However, Queen Isabella of Spain was pictured in the Columbus series.
Martha Washington.
MIRABAI
The very first postage stamp in the world pictured a woman, namely Queen Victoria in 1840. Except for some allegorical figures and namely Indian women, Queen Isabella of Spain was the first woman to be seen on US stamps.
Harriet Tubman was the first to have her own stamp. Some black female postal workers appeared on an earlier stamp.
My guess is Empress Zewditu of Ethiopia who appeared on a 1918 stamp of Ethiopia. You didn't ask from what country, so it's hard to say. If you are talking about the United States, the first Black woman to appear on a postage stamp was Harriet Tubman on Feb 1, 1978.
Great Britain issued the first stamps in May of 1840. They pictured Queen Victoria on them.
Snehamol
Martha Washington, wife of President George Washington.