Six-time champion Rea, who has made the transition from Kawasaki to replace BMW-bound Toprak Razgatlioglu, turned 12 laps aboard the R1 he will race in 2024 on a rain-affected opening day of testing.
He posted a best time of 1m40.302s to go fourth-fastest, a little under half a second shy of pacesetter Remy Gardner, although his day was interrupted by a crash at Turn 13.
“It was fantastic to get the first laps on the R1 because you always have anticipation and maybe a preconception of how the bike’s going to be," Rea told the official WSBK website.
"I’ve been super excited until this morning, and then I got really nervous all of a sudden. I was thinking ‘can I be fast with the bike?’ or ‘what’s it going to feel like sitting on the bike during the bike fit?’.
"I got comfortable but the position felt strange, and I couldn’t imagine riding the bike. As soon as I exited pitlane, there were a lot of positives straight away I could take.
"I think there’s still a lot of margin with the bike. It’s user-friendly which gives me a good feeling."
GRT Yamaha rider Gardner led Tuesday's running with a best time of 1m39.837s, albeit with bad weather effectively limiting the day's action to just the morning.
Second-quickest was Michael Ruben Rinaldi, whose move to the satellite Motocorsa Ducati squad was confirmed on Monday following last weekend's season finale, just under two tenths slower.
Dominique Aegerter made it both GRT Yamaha riders in the top three, around two tenths faster than Rea.
As well as Rea, newly-crowned two-time WSBK champion Alvaro Bautista also suffered a crash at Turn 13 on his way to the fifth-fastest time of the day.
Bautista's new team-mate Nicolo Bulega was eighth-fastest behind Garrett Gerloff on the Bonovo BMW and Alex Lowes on the best of the Kawasakis.
Axel Bassani had his first taste of Kawasaki machinery on Tuesday as he took over Rea's seat at the team, going 10th-fastest.
Scott Redding meanwhile was ninth on his first day of action at the Bonovo BMW team after losing his place at the factory team to Razgatlioglu for 2024.
Ex-MotoGP racer Andrea Iannone was also in action for the Go Eleven Ducati team as he gears up for his return to racing next year once his four-year doping ban finally expires.
The Italian rider was 11th of the 13 riders present, a little over two seconds off the pace.
Britons Bradley Ray (GMT Yamaha) and Tarran Mackenzie (MIE Honda) made up the 13-strong field.
While Rea was able to get his first taste of Yamaha machinery on Tuesday, Razgatlioglu will have to wait until December to sample the BMW with Yamaha refusing to release the Turkish rider from his contract early.
Post-season testing continues at Jerez on Wednesday.
Jerez World Superbike test times (Tuesday):
1. Remy Gardner (GRT Yamaha) - 1:39.837s (25 laps)
2. Michael Ruben Rinaldi (Motocorsa Ducati) +0.197s (36)
3. Dominique Aegerter (GRT Yamaha) +0.269s (22)
4. Jonathan Rea (Yamaha) +0.465s (12)
5. Alvaro Bautista (Ducati) +0.727s (24)
6. Garrett Gerloff (Bonovo BMW) +1.356s (12)
7. Alex Lowes (Kawasaki) +1.392s (14)
8. Nicolo Bulega (Ducati) +1.407s (24)
9. Scott Redding (Bonovo BMW) +1.496s (20)
10. Axel Bassani (Kawasaki) +1.964s (27)
11. Andrea Iannone (Go Eleven Ducati) +2.085s (26)
12. Bradley Ray (GMT94 Yamaha) +2.458s (27)
13. Tarran Mackenzie (MIE Honda) +4.429s (7)
