Combine pineapple, onion, jalapeno, tomatoes, cilantro, garlic, salt, and chili powder in a large bowl and stir to combine.
Lightly grease your slow cooker. Place chicken in a single layer (the best you can, it's okay if they overlap a little) in the bottom of the slow cooker.
Pour salsa mixture over chicken.
Cover and cook on low 3-4 hours or on high 2-3 hours. Serve chicken and salsa over rice if desired. (see note for alternate serving option)
Notes
Another great way to serve this chicken is shredded, in tacos. After cooking, use two forks to pull the chicken apart til shredded and stir to combine salsa and chicken. Serve in flour or corn tortillas.