Arriving at Valderrama off the back of a tied 17th finish at the Acciona Open de España in Madrid last week, Rahm was unable to thrill the Spanish crowd with four bogeys in his first six holes.
He added a double-bogey at the par-4 18th hole, which was his ninth hole of the day. After playing nine holes in 42 shots, he managed to steady the ship with six consecutive pars at the start of his second nine holes.
However, the US Open champion made another bogey at the par-4 7th hole and parred the final two holes to finish with a 7-over-par 78.
This is the worst score that Rahm has shot in 2021 on any tour with his previous low being a 76. His 78 also ties the worst score he has recorded since turning professional in 2016.
Scoring was drastically higher when compared to last week's tournament in Madrid. The tight fairways, narrow tree lines and high winds of this former Ryder Cup venue were proving hard to negotiate.
When Rahm finished his round, he was 11 shots behind leader Julien Guerrier who shot 4-under-par on Thursday. The Frenchman made six birdies and two bogeys to set the early mark in the clubhouse.
Rafa Cabrera-Bello, winner of the Acciona Open de España last week, shot a superb 3-under-par 68 and Matthew Fitzpatrick scored a level-par 71.
Englishman Andy Sullivan also severely struggled on the first day at Valderrama. He carded five bogeys and two double-bogeys on his way to an uncharacteristic 80.
The popular Eddie Pepperell struggled to a 7-over-par 78 and Betfred British Masters champion Richard Bland shot a 4-over-par 75.