Rahm, one of 13 LIV players in the year's first major and bidding to become the fourth golfer to successfully defend his Masters crown, found the fairway with his opening tee shot and two-putted from 16 yards after his approach settled on the fringe.
For Rahm, it was a much steadier start to the Masters than last year when he carded a four-putt double bogey at the par-four opening hole, a 445-yard dogleg right that plays uphill.
Playing one group behind Rahm was a high-profile threesome featuring world number one and tournament favourite Scottie Scheffler, Grand Slam-seeking Rory McIlroy and Olympic champion Xander Schauffele.
Storms delayed one of the most anticipated days in golf by 2-1/2 hours but the overnight rain left the greens much softer than they were during practice rounds and, with little wind, presented plenty of opportunities for the early starters.
The delay means the day's late starters, including five-times champion Tiger Woods, 2023 runner-up Brooks Koepka and former champion Dustin Johnson, are unlikely to finish their first rounds on Thursday.
Big-hitting DeChambeau took full advantage of the softened conditions as the 2020 U.S. Open champion strung together three consecutive birdie putts, none longer than six feet, to start his round as the sun started to peek through the clouds.
South Africa's Erik van Rooyen, former Masters winner Danny Willett, amateur Christo Lamprecht and New Zealand's Ryan Fox were all level with DeChambeau.
Woods, making only his second start this year due to nagging injuries, is scheduled to tee off at 15:54 ET (20:54 CET) in the fourth-last group with Australian Jason Day and Max Homa.
According to the weather forecast, there is no more than a 30% chance of showers for the duration of the first round but high winds are expected with peak gusts in the 35-40 mph range possible for the rest of the day.
The forecast for the next three days is for sunny skies and warmer temperatures.
