Petrus ("Piet") Johannes Keizer (born June 14, 1943 in Amsterdam) was a Dutch professional football player. Having played 364 matches for Ajax, from 1961 to 1974, he is the fourth most capped player for Ajax (in terms of league appearances). He played on left-wing and with Ajax won the European Cup three consecutive times (1971, 1972, 1973), having lost the 1969 final to A.C. Milan.
With the Dutch national team, Keizer played 34 times, scoring 11 goals. He was in the Dutch squad for the 1974 FIFA World Cup.