Gather the ingredients.
Select the Instant Pot's sauté setting and add the oil. When the oil is hot and shimmering, add the ground beef. Cook for 4 minutes, stirring constantly.
Add the onion, bell pepper, and garlic to the beef and cook for 2 to 3 minutes longer, stirring frequently.
Add the water to the pot and stir, scraping the bottom of the pot to loosen any stuck-on browned bits.
In a bowl, combine the ketchup, brown sugar, Worcestershire sauce, and mustard. Add the mixture to the Instant Pot; do not stir.
Lock the lid in place and turn the steam release knob to the sealing position. Choose the pressure cook or manual setting (high pressure) and set the timer for 10 minutes. When the time is up, let the pressure come down naturally for 10 minutes and then carefully do a quick release following the manufacturer's instructions.
Cancel the pressure cook function and choose the sauté button. Continue cooking the sloppy joe mixture for 2 to 3 minutes, or until reduced and thickened.
Spoon the sloppy joe sauce over toasted buns and serve with chips, fries, or tater tots.