Place the chicken breasts in a gallon zip-top bag and, using a meat mallet, pound each breast to an even thickness.
In a bowl, whisk together the Italian dressing and the taco seasoning until well combined.
Pour the dressing mixture into the bag with the chicken, seal, and refrigerate for at least 1 hour or up to overnight to marinate.
Remove the chicken from the marinade and discard the excess marinade.
Preheat your grill (or cast iron grill pan) to medium-high heat. Grill the chicken 6–8 minutes per side, or until an instant-read thermometer inserted into the thickest part reads 165°F.
Transfer the chicken to a plate, let rest a few minutes, then serve.