Mr Trump was impeached by the US House of Representatives last month on charges of abuse of power and obstruction of Congress.
For just the third time in history, the Senate has opened an impeachment trial to determine if a sitting president should be removed from office.
Mr Trump has denied these claims, calling the impeachment proceedings a partisan "hoax".
Eventual acquittal is expected in the Republican-controlled Senate.
Starr stepped down as president of Baylor in 2016 following an investigation by the school's board of regents into allegations that he failed to respond to a sexual assault crisis involving the school's football team.
Dershowitz is among the most well-known attorneys in the country.
Dershowitz posted on Twitter that he "will present oral arguments at the Senate trial to address the constitutional arguments against impeachment and removal".
Among the additions to the President's defence campaign is Ken Starr, the prosecutor whose investigation led to Bill Clinton's impeachment, and Alan Dershowitz, the constitutional lawyer who once represented sex offender Jeffrey Epstein.
"He is participating in this impeachment trial to defend the integrity of the Constitution and to prevent the creation of a risky constitutional precedent", Dershowitz's team wrote on Friday.
Starr succeeded Robert Fiske as independent counsel investigating Bill and Hillary Clinton's past real estate dealings in the mid-1990s.
One person who was not added to the team that will defend Trump at the trial is his personal lawyer Rudy Giuliani, who played a key role in the Ukraine matter.
Earlier on Thursday, House Democrats prosecuting the case stood before the Senate and formally read the articles of impeachment against Trump. Schiff is a former federal prosecutor in Los Angeles.
Following a White House directive, several key figures in the Trump administration's dealings with Ukraine refused to provide testimony or documents in the House impeachment inquiry.
Later Chief Justice John Roberts, who is to preside at the trial, will administer the oath to senators who will serve as jurors and swear to deliver "impartial justice". I am posting the statement Professor Dershowitz has released via Twitter below. Lofgren has the rare credential of having worked on a congressional staff during President Richard Nixon's impeachment - he resigned before the full House voted on the charges - and then being an elected lawmaker during Clinton's.