With the Italian national team, Castellazzi made three appearances between 1929 and 1934; he made his international debut on 1 December 1929, in a 6–1 friendly home win over Portugal in Milan, and later appeared in a 4–2 friendly home win over Switzerland in Rome, on 9 February 1930. He also made one appearance – his final international cap – in the team's victorious 1934 FIFA World Cup campaign on home soil, starting in the first quarter-final against Spain, held in Florence, on 31 May, which ended in a 1–1 draw.