Michael Joseph Stack III is an American attorney and former politician who served as the 33rd lieutenant governor of Pennsylvania from 2015 to 2019. A member of the Democratic Party, he previously served as a member of the Pennsylvania State Senate for the 5th district from 2001 to 2015.

The lieutenant governor is a constitutional officer of the Commonwealth of Pennsylvania. The lieutenant governor is elected for a four-year term in the same year as the governor. Each party picks a candidate for lieutenant governor independently of the gubernatorial primary. The winners of the party primaries are then teamed together as a single ticket for the fall general election. The lieutenant governor presides in the Pennsylvania State Senate and is first in the line of succession to the governor; in the event the governor dies, resigns, or otherwise leaves office, the lieutenant governor becomes governor. The Lieutenant Governor casts tie breaking votes in the State Senate.