The 38-year-old superstar, retiring from professional tennis after the tournament, was selected by captain David Ferrer to face Botic van de Zandschulp in Malaga.
The Davis Cup will be the final professional tournament of Nadal's glittering career of almost 23 years.
The 22-time Grand Slam winner has battled injuries over the past few years, with his fitness struggles ultimately leading him to the decision to retire.
Doubts remained over whether the clay court king would be selected by Ferrer for one of the rubbers until the official announcement on Tuesday afternoon.
Nadal has not competed in an official singles match since the Paris Olympics in July, where he was beaten in the second round.
Record 14-time French Open champion Nadal said Monday he was not in Malaga to retire but instead to try and help Spain win the Davis Cup for the first time since 2019.
Nadal has a 29-1 win-loss record in the Davis Cup, where he first appeared for Spain in 2004.
Tennis great Roger Federer hailed his former rival before the match, saying Nadal had "made Spain proud... you made the whole tennis world proud".
