The Lotus driver had brought the morning session to a premature halt when his car stopped out on the circuit, but he ended up fastest with a 1m22.716s lap on soft tyres in the afternoon.
Jenson Button's McLaren had set the pace in the morning and he looked strong for most of the afternoon.
With the day starting wet and gradually drying until a shower at the very end, Button's morning time was soon surpassed in the afternoon.
But runs on the medium-compound slick Pirellis kept Button near the top of the timesheets and his 1m23.181s was the day's best medium-tyred time.
Pastor Maldonado's Williams jumped to third in the closing minutes, just getting his run in before the rain, which has dogged Barcelona testing this year, returned.
That left him ahead of Red Bull's Sebastian Vettel, who also made a late improvement, and the Sauber of Nico Hulkenberg in fifth.
Adrian Sutil, recently confirmed as Force India's second driver, actually topped the times after lunch, but the squad elected not to do as much running later and he slipped to sixth.
Nico Rosberg (Mercedes) and Ferrari's Fernando Alonso concentrated on longer runs, with Alonso in particular running consecutive stints on the hard tyre when the track was at its best. That meant he ended up at the bottom of the times.
An otherwise clean session was interrupted twice in the last 20 minutes. First, Giedo van der Garde's Caterham stopped and then, after the restart, Maldonado's Williams brought the day to a slightly early conclusion.
Results:Pos Driver Team Time Gap Laps
1. Romain Grosjean Lotus 1m22.716s 88
2. Jenson Button McLaren 1m23.181s + 0.465s 72
3. Pastor Maldonado Williams 1m23.628s + 0.912s 75
4. Sebastian Vettel Red Bull 1m23.743s + 1.027s 65
5. Nico Hulkenberg Sauber 1m23.744s + 1.028s 79
6. Adrian Sutil Force India 1m24.215s + 1.499s 62
7. Daniel Ricciardo Toro Rosso 1m25.483s + 2.767s 61
8. Max Chilton Marussia 1m25.598s + 2.882s 75
9. Giedo van der Garde Caterham 1m26.316s + 3.600s 48
10. Nico Rosberg Mercedes 1m26.655s + 3.939s 120
11. Fernando Alonso Ferrari 1m27.878s + 5.162s 102
- Motor Racing
- Sports & Recreation
- Jenson Button
- Pastor Maldonado