The Mavericks open the regular season at the Phoenix Suns on Wednesday.
The 31-year-old Campazzo became a free agent after spending his first two NBA seasons in Denver, following careers in the EuroLeague and his native Argentina.
Campazzo played alongside Doncic at Real Madrid and helped the Spanish club win championships in 2015 and 2018. Doncic was MVP of the EuroLeague Final Four for the second title.
The 5-foot-11 Campazzo averaged 5.6 points and 3.5 assists in two seasons with the Nuggets, starting 23 of 130 games.
Campazzo started his professional career at 17 in his home country and was a two-time Argentine League Finals MVP while helping his team win four championships in five seasons.
The addition of Campazzo comes as the Mavericks figure out their rotation following the departure of point guard Jalen Brunson.
Brunson started his career as a backup to Doncic but had a stellar postseason performance last season as a starter alongside the Slovenian standout. Brunson signed with the New York Knicks in free agency.
Campazzo is captain of Argentina's national team, which won the FIBA AmeriCup gold medal last month. He averaged 12.3 points in the tournament.