The double world champion, who joined Aston Martin this season, was only ninth fastest in qualifying and was beaten for the first time by Canadian teammate Lance Stroll (24), who starts fifth, up from sixth, after Pierre Gasly (27) was given a six-place grid penalty for impeding both Max Verstappen (25) and Carlos Sainz (28) during the session.
Alonso, whose 32nd and last career win was at Barcelona's Circuit de Catalunya with Ferrari in 2013, had started the two previous races in Miami and Monaco on the front row - finishing the latter second.
He damaged his car's floor by going off into the gravel on Saturday, and reckoned that had maybe cost him a couple of tenths.
He also went on the damp part of the track at turn 10 on his final flying lap.
"It was not my best Saturday, hopefully a good Sunday tomorrow," he said.
"Maybe the podium is a little bit too far away at the moment, starting P9, but top five I think is something we should target.
"We have the pace, we have a good (tyre) degradation and let's see with the new layout if it's easier or worse to overtake and what we can do with the strategy."
Alonso's form, with five podium appearances in six races, has created a buzz reminiscent of his glory days at the Barcelona circuit.
"It’s the whole circuit and the whole of Barcelona. Wherever you go, you see the green (Aston Martin) shirts," said team boss Mike Krack on Friday.
"In front of our garage yesterday it was just amazing what happened there. And this is obviously translating into the whole team and it’s just fantastic to see something like that."