Combine the rice and pesto and season generously with salt and pepper. Divide into 20 equal sized balls. Cut the mozzarella into 20 small pieces. Push the cheese into each ball and squeeze tightly to enclose.
Heat the oil in a saucepan to 350 degrees. Fry the rice arancini, a few at a time, until deeply golden brown, about 90 seconds. Remove with a slotted spoon, sprinkle with a bit of salt and repeat with the remaining arancini.
Serve warm with warm marinara sauce for dipping.