Pence, a staunchly conservative 57-year-old, served six terms in Congress before being elected governor and could help Trump navigate Capitol Hill.
Trump went back and forth between touting Pence's accomplishments as IN governor and attacking presumptive Democratic nominee Hillary Clinton.
Republican sources said on Thursday that Trump had decided on Pence, but the campaign had not confirmed this until now.
Donald Trump said Saturday that Indiana Gov. Mike Pence will be "my partner" in a fight to change the United States by rebuilding the manufacturing sector at home and fighting terrorism from overseas.
"Let's come together as party, as people and as a movement to make America great again", Pence said.
The event set to begin at 11 a.m. follows Trump, the presumed Republican presidential candidate, publicly announcing Pence's selection in a tweet Friday and caps off a whirlwind week for the governor.
The reaction to the Pence choice from Republican officials was overwhelmingly positive - no small feat for Mr Trump, given how polarising he has been within his own party. But he could help unify Republicans left divided by Trump's campaign to win the party presidential nomination for the November 8 election.
Pence would have the backing of GOP leaders and ease some of their concerns about Trump's political inexperience and volatile temperament.
No comedian can do or say anything more hilarious than the gold that Trump and his campaign pump out multiple times a day.
'Donald Trump just doubled down on his agenda of hate and discrimination, ' said HRC President Chad Griffin.
In an interview with Sean Hannity of Fox News, Pence downplayed or reversed past differences he has had with Trump.
"The reason anyone in America could pick Mike Pence out of a one man lineup is because of his long history of targeted political attacks against the care provided at Planned Parenthood health centers in IN and across the country". But some of Trump's children, who have been closely advising their father, were said to favor different candidates.
He has often described himself as "a Christian, a conservative and a Republican, in that order".
The Clinton campaign said in a statement that Pence is "the most extreme Vice Presidential pick in a generation", and posed the question: "Think Donald is divisive?" With Pence sitting in a NY hotel, Trump made a decision to postpone the announcement. Pence's demeanor is as calm as Trump's is fiery and he brings a sense of discipline that aides and advisers hope can bridge that gap.
The billionaire commented on Pence's earlier endorsement of Cruz, joking about how he also praised Trump During Cruz's endorsement.
In addition to Pence, Trump's shortlist included former House Speaker Newt Gingrich and New Jersey Gov. Chris Christie, according to people familiar with the candidate's thinking.