The 47-year-old Republican took his seat after a swearing-in ceremony led by Vice President Joe Biden. GOP Gov. Chris Christie picked his attorney general to serve as interim senator until after an Oct. 16 special election.
The seat opened up last week after the death of Frank Lautenberg, 89, a liberal Democrat who served nearly 30 years in the Senate. With Chiesa taking his seat, the Senate has 52 Democrats, 46 Republicans and two independents who often vote with the Democrats.
"I have work to do to learn about the institution and about the issues," said Chiesa. "I'm getting my legs steadied underneath me and I'll do everything I can to fulfill my obligation to the people that I represent."
A longtime Christie colleague, Chiesa has never held or run for political office and will not seek Lautenberg's former seat in the special election. The special election in October is to fill the seat until the term expires in January 2015. Whoever wins in October would have to run again in the fall 2014 for a full six-year Senate term.
A primary is set for Aug. 13. New Jersey congressmen Rush Holt and Frank Pallone are in the Democratic race along with Newark Mayor Cory Booker and state Assembly Speaker Sheila Oliver.
Steve Lonegan, a conservative former mayor of Bogota who has twice lost gubernatorial primaries, and Somerset physician Alieta Eck are running in the Republican primary.
Chiesa arrives as the Senate tackles a bipartisan immigration overhaul this week that would offer a path to citizenship for millions of immigrants living in the country illegally. He's described himself as a conservative who favors putting border security first on immigration.
"Senator Chiesa comes at a time in which we are having some momentous debates in this nation," said Sen. Robert Menendez, D-N.J.
Menendez added that Chiesa would be casting some "critical votes" in the coming months and that he looked forward to working with him.
Senate Minority Leader Mitch McConnell, R-Ky., called Christie's pick of Chiesa a "wise appointment."
Chiesa served as executive director of governor's transition team after Christie's 2009 election. He was Christie's chief counsel until he was nominated to be the state's attorney general in December 2011. Chiesa as attorney general ran an extensive series of gun buybacks across the state. He also initiated a statewide taskforce to address human trafficking and oversaw statewide child-pornography stings.
Chiesa will be the first Republican to represent New Jersey in the Senate since 1982 when Nicholas Brady was chosen by Gov. Tom Kean to serve out a term after Harrison Williams resigned amid scandal. New Jersey has not elected a Republican to the Senate since 1972.