Alaska officially became a state on January 3, 1959. When this happened, it became the 49th state in the United States. Later that year, Hawaii became the 50th and (so far) final state in the Union.
Unlike most parts of the United States, Alaska did not make an easy transition from being a territory to becoming a state. Many people in the US did not want Alaska to have that sort of official status for two reasons. First, it was not contiguous to the rest of the United States. Second, much of its population was not white. For these reasons, it faced more difficulties than some states in achieving statehood. Eventually, though, resistance lessened. President Eisenhower backed statehood, which helped break down resistance in Congress. Alaska then became a state on January 3, 1959.