Alonso's time of 1:21.875 was just shy of three tenths of a second quicker than the Sauber of Nico Hulkenberg.
Most of the quickest lap times were set during the morning session as most of the eleven teams concentrated on race simulations during the afternoon.
Romain Grosjean was in third place and was the busiest driver, completing 119 laps. The Frenchman completed a race simulation of 66 laps in the afternoon as Lotus made up for lost time after problems struck Kimi Raikkonen on the opening two days of the test.
Nico Rosberg was fourth in the Mercedes W04 as he also completed over 100 laps at the home of the Spanish Grand Prix.
Pastor Maldonado drove the Williams FW35 in the morning before handing over to Valtteri Bottas for the afternoon session.
Jenson Button was seventh, ahead of the returning Adrian Sutil in the Force India VJM06.
Sutil, driving an F1 car for the first time since the 2011 Brazilian Grand Prix, acquitted himself well, his fastest time being just over a second slower than Alonso. 2012 reserve driver Jules Bianchi takes over from Sutil tomorrow.
Mark Webber, Jean Eric Vergne, Max Chilton and Giedo van der Garde completed the order. The Dutchman caused a red flag fifteen minutes from the end of running as he stopped his Caterham CT03 on the entry to Turn 3.
The test concludes tomorrow with Brazilians Felipe Massa and Luiz Razia set to get their first runs of the week. Lewis Hamilton and Esteban Gutierrez will also return to the track, although heavy rain is forecast to hit the circuit throughout the day.
|1||F. Alonso||Ferrari F138||1:21.875||96|
|2||N. Hulkenberg||Sauber C32||1:22.160||91|
|3||R. Grosjean||Lotus E21||1:22.188||119|
|4||N. Rosberg||Mercedes W04||1:22.611||106|
|5||P. Maldonado||Williams FW35||1:22.675||79|
|6||V. Bottas||Williams FW35||1:22.826||66|
|7||J. Button||McLaren MP4-28||1:22.840||71|
|8||A. Sutil||Force India VJM06||1:22.877||78|
|9||M. Webber||Red Bull RB9||1:23.024||106|
|10||J. Vergne||Toro Rosso STR8||1:23.366||106|
|11||M. Chilton||Marussia MR02||1:25.690||56|
|12||G. van der Garde||Caterham CT03||1:26.177||93|