Cook the bacon in a large skillet over medium heat until crispy. Transfer to a paper towel-lined plate to drain any grease. Once cool enough to handle, finely chop the bacon. You should have about 2 tablespoons.
Mix the bacon, jalapeno, mayo, and ketchup in a medium bowl until combined. Season with salt and pepper to taste.
Serve immediately as a dip for fries or onion rings or as a spread for sandwiches and burgers. Store leftovers in an airtight container in the fridge.
Notes
If you like your fry sauce spicier, you can add a second jalapeno for a total of 2 tablespoons. You can also leave the seeds in the sauce for an extra kick.