The 24-year-old is capped by Austria at Under-21 level, and has been on the Hornets' books since last summer.
Bachmann was previously at Stoke City, where he spent time on loan at Wrexham, Ross County, and Bury between 2014 and 2016.
"It's a bit of a change, and I'm looking forward to starting playing," Bachmann told official Kilmarnock website.
"It's a bit of a difference but it looks nice and green around here and I'm sure I will settle quickly. I just need to get used to the Scottish accent!
"It's very different for me, but it's quite funny and I'm getting used to it. I've enjoyed the first couple of training sessions and I'm looking forward to the rest of the season.
"The biggest part is to play and help the team do well and finish as high as we can this season."