The young Monegasque's time of one minute 16.231 seconds was a talking point on a sunny day at the Circuit de Catalunya, confirming Ferrari's status as early frontrunners, while Frenchman Pierre Gasly had a heavy crash in his Red Bull.
Lapping in ambient temperatures of 21 degrees Celsius, Leclerc went more than half a second faster than the best of the rest on the softest, and fastest, C5 compound tyres.
He later stopped on track with an exhaust issue but the overall tone emanating from the team was upbeat.
Asked by a reporter whether he would put his own money on the Italian team if he had to bet on who was fastest, Leclerc replied: "At the moment I'm happy, so yes."
The lap was close to the 2018 Spanish Grand Prix pole time of 1:16.173 set by Lewis Hamilton for Mercedes, and came as a boost for Ferrari after Sebastian Vettel had crashed on Wednesday.
The penultimate day of testing, before teams head back to their factories and on to Australia for the March 17 season-opener, saw another high-speed incident when Gasly went off at turn nine.
The Frenchman, who has moved up from Toro Rosso to replace departed Australian Daniel Ricciardo, was unhurt but the impact did considerable damage to the car and ended the team's programme for the day.
While Red Bull have impressed with their new Honda engine, Gasly said Ferrari were still ahead.
"I think they are really fast, they showed some good pace," he said.
Hamilton's team mate Valtteri Bottas reiterated the view he expressed last week that Ferrari were ahead and Mercedes still had work to do.
"The whole field looks like it's a bit tighter than last year or the year before, but it's only guessing," added the Finn. "You never know what people bring to Melbourne."
The season-opening race in Australia is on March 17.
Anglo-Thai driver Alexander Albon was second fastest with a lap of 1:16.882, also on C5 tyres, with McLaren's Lando Norris third in 1:17.084.
Norris's team mate Carlos Sainz, who went quickest on Wednesday, played down the times.
"This is just testing and we are just doing more qualifying-based runs so we can see what you can do with the car. I guess we need to wait and see what the others can do on those runs," said the Spaniard.
Hamilton, who will be chasing a sixth title this year with Mercedes going for a sixth successive championship double, did more laps than anyone before lunch (85) but was eighth fastest on the hard C2 tyre as he focused mainly on long runs.
Bottas did 96 laps after lunch but his best effort of 1:18.862 was still the slowest time of the day.
Leclerc did 138 laps in total, Norris 84 and Albon 118. British rookie George Russell put in a hefty shift for Williams, completing 140 laps.
(Reporting by Alan Baldwin in London, editing by Ed Osmond and Toby Davis)
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