Yes, you can, but only the ACT Mathematics Test. It's not required, however, because the problems can be solved without one. However, there's some prohibited calculators:
- Calculators with built-in computer algebra systems
- Cell phone calculators
- Calculators with the QWERTY format keypad
- Computers/PDAs/tablets and all that
- Electronic writing pads
Hope this is helpful, but still, I'd check act.org's FAQ just to be sure!