Both players will join the United States' World Cup preparation camp on Thursday at Stanford University in Palo Alto, California.
The United States' final 23-player roster for the 2014 FIFA World Cup - to be held from June 12 - July 13 in Brazil - is limited to players on the preliminary roster and must be submitted to FIFA by June 2.
Besler, 27, has made 14 appearances for the United States since debuting in January 2013, including six FIFA World Cup qualifiers and all three knockout round games en route to the 2013 CONCACAF Gold Cup championship.
He most recently appeared alongside Zusi in the United States' 2-2 draw against Mexico on April 2, and the U.S. is 9-1-3 with Besler in the starting lineup.
The Overland Park, Kan. native was named Sporting Kansas City's captain prior to the 2014 season, his sixth as a professional after being selected eighth overall in the 2009 MLS SuperDraft out of Notre Dame.
Besler has appeared in 144 MLS matches (including postseason) in his career, earning MLS Best XI (2012, 2013), MLS All-Star (2011, 2013) and MLS Defender of the Year (2012) honors.
"I am excited to join the United States for the final training camp before the World Cup," Besler said. "It's a big honor and I always take pride in playing for the national team. That being said, there is still a lot of work to be done and I look forward to getting to camp and starting the next phase of World Cup preparation."
Zusi has earned 20 appearances for the United States with three goals and four assists in his international career. He scored in the final two FIFA World Cup Qualifiers, highlighted by the game-winning goal against Jamaica at Sporting Park, to secure a first place finish in the Hexagonal for the United States.
Over six professional seasons, the 27-year-old playmaker has played in 141 MLS matches (including postseason) with 18 goals and 40 assists. Zusi recorded a career-high six goals and a team-best 12 assists in all competition in 2013 and was the top-rated midfielder in the MLS Castrol Index.
The Maryland alum was an MLS MVP finalist in 2012 and nominee for Best MLS Player at the 2013 ESPYS to go along with MLS All-Star (2012, 2013) and MLS Best XI (2012, 2013) recognition.
"It's a tremendous honor to represent your country at any point in your career and especially now to have that chance on the cusp of a World Cup," Zusi said. "Everyone involved within the national team worked extremely hard through qualifying to earn the opportunity in front of us and now the training camp is vitally important in this final stage of preparation."
The United States will train at Stanford University leading up to their first match of the 2014 Sendoff Series on May 27 against Azerbaijan at Candlestick Park in San Francisco, California.
The series will continue with friendlies against Turkey (June 1 at Red Bull Arena in Harrison, New Jersey) and Nigeria (June 7 at EverBank Field in Jacksonville, Florida) before the team arrives in Brazil on June 9.
"We had some difficult decisions to make, and we are very excited about the 30 players coming into camp," United States head coach Jurgen Klinsmann said. "There is competition in every position, and these next couple of weeks will allow us to make the final determinations on the best 23-man unit to represent the United States in the World Cup."
Ranked 14th in the latest FIFA rankings, the United States was drawn in Group G of the 2014 FIFA World Cup alongside Germany (No. 2), Portugal (No. 3) and Ghana (No. 38). This summer's competition will be the United States' seventh straight World Cup and 10th overall appearance in the world's most widely viewed sporting event.