Who was the first U.S. President to visit all 50 states?

Richard Nixon actually visited all 50 states while campaigning for president when he lost to John F Kennedy.

https://en.wikipedia.org/wiki/Fifty-state_strategy

Once Nixon was in office he actually visited all 50 states as president. George H W Bush, Bill Clinton, and Barak Obama have also visited all 50 states.

https://news.google.com/newspapers?nid=1298&dat=19711006&id=C_RNAAAAIBAJ&sjid=14oDAAAAIBAJ&pg=7180,4532177

Let's not forget that Hawaii joined the U.S. in 1959 to become the 50th state, so President Eisenhower (1953-1961) was technically the first to be able to visit all 50 states (which he didn't do). Earlier presidents just didn't have 50 states to visit.

However, if we ask the question which presidents visited each state of the U.S. as it was by the time of their respective stint in the White House, the somewhat surprising answer is: only Washington and FDR.

The first president to visit every single state of the U.S. during his tenure was George Washington. He did this within two and a half years between 1789 and 1791. You might say that visiting 13 states all along the East Coast is not a remarkable feat but keep in mind that Washington was traveling on horseback and by carriage along rutted dirt roads which often were little more than bumpy paths through the wilderness. Jefferson, for example, visited only five.

The next president to visit all states was Franklin Roosevelt showing up in 48 states during his twelve-year presidency. President Hoover's travels within the U.S. are not completely recorded but he didn't exceed 40 states.

Sources:
https://www.mountvernon.org/library/digitalhistory/digital-encyclopedia/article/george-washington-and-travel/
https://www.mentalfloss.com/article/63274/how-many-presidents-visited-all-50-states-leaving-office
https://www.monticello.org/research-education/thomas-jefferson-encyclopedia/extent-jeffersons-travels/