The Finn will share the third row with his former team-mate, Lewis Hamilton, and three places clear of George Russell, the man who replaced him at Mercedes.
All new regulations for 2022 promised a mixed up pecking order, with qualifying offering clues as to the shape at least the early part of the season will take.
“P6 is good for us. I think we did know that, if we get everything right, we have a chance to get to Q3, but it was extremely close,” Bottas said.
“I got some good laps in there and [my] Q3 lap I was really, really pleased with that.”
Alfa Romeo Sauber comes off the back of a disappointing 2021 campaign in which is scored just 13 points and finished ninth in the constructors' championship.
Saturday's performance in Bahrain is therefore something of a morale booster for the Swiss-squad and, according to Bottas, there's reason to be optimistic heading into the race.
“I'm proud of the team, everyone in the team and also in the factory,” said the Finn.
“I think our race pace, if anything, it has looked actually better than single lap pace.
The Bahrain Grand Prix, the opening round of the 2022 F1 season, starts at 02:00 AEDT on Monday.