Massaro made his Italy under-21 debut on 23 September 1981, 10 days after his Serie A debut with Fiorentina. Overall, he made 4 appearances with the "Azzurrini" between 1981 and 1984, also taking part with Italy's Olympic under-23 side at the 1984 Olympics, where Italy reached the semi-final, finishing the tournament in fourth place. Surprisingly capped only 15 times for the Italian senior side, Massaro's international career actually spanned more than a decade between 1982 and 1994. As a 21-year-old, Massaro made his debut on 14 April 1982 under Enzo Bearzot in a 1–0 defeat to East Germany, and he was a member of the Italian squad that won the 1982 FIFA World Cup held in Spain, although he did not receive any playing time during the tournament. He was capped sparingly between 1984 and 1986, but eight years later he was called up for Italy's 1994 World Cup squad by manager Arrigo Sacchi, at the age of 33. He played in six of Italy's seven games at the 1994 FIFA World Cup held in the United States, and scored a goal in a 1–1 draw against Mexico in Italy's final match of the group stage on 28 June, which allowed them to progress to the knockout round as the best third-placed team; this was his only goal for Italy, and made him Italy's oldest ever goalscorer at the FIFA World Cup, at the age of 33 years and 36 days. In the defeat against Brazil in the final of the tournament, he missed a one-on-one opportunity and later failed to convert a penalty kick in the shoot-out.