It's the 18th different team he's managed and the 10th club in Italy.
Ranieri replaces Eusebio Di Francesco, who left Sampdoria after six defeats in seven matches.
The 67-year-old has now supplanted Di Francesco twice in consecutive seasons. He returned to coach hometown side Roma on an interim basis following Di Francesco's sacking in March.
Roma ended the 2018-19 campaign in sixth place, three points short of a Champions League spot, and allowed Ranieri to leave.
Ranieri's task is now very much about survival. Following a slew of departures - including the sale of Danish bedrock Joachim Andersen - Sampdoria have struggled to keep the ball out of the net. No other team in Serie A has conceded more than the Blucerchiati's 16 goals.