Great Britain issued the first stamps in May of 1840. They pictured Queen Victoria on them.
Martha Washington.
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.
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.
The first American woman to be featured on a US stamp is Martha Washington. The stamp was released in America in 1902.
In the US I believe it is Scott Number 2137 issued in 1985. It pictured Mary McLeod Bethune, Educator.
Martha Washington, wife of President George Washington.
St. Alphonsamma is the first woman from Kerala to be pictured on the Indian postal stamp.
MIRABAI
Scott 234 241 and 244 No, it was Queen Isabella of Spain in the Columbian Exposition series of stamps. She appeared twice in that series of stamps.
Harriet Tubman
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.