The brother of Galaxy midfielder Jose Villarreal, Jaime Villarreal, 21, joins the club after making 61 appearances over two and a half seasons in the USL with LA Galaxy II.
"Jaime is a player that has grown up in our system while excelling with LA Galaxy II," Galaxy general manager Pete Vagenas said in a news release. "We are confident that success can continue with the first team and we are excited to add him to our roster."
Smith, 22, skipped his final season at UCLA to sign with LA Galaxy II last season, making 24 appearances. In 2011, he represented the U.S. at the U-17 World Cup.
"Nathan is a player that has shown promise during his time with LA Galaxy II," Vagenas said. "His signing with our first team is a testament to his continued growth on the field. We think he can continue to develop to contribute to our team at the MLS level."
Villarreal and Smith, who both played under new Galaxy coach Curt Onalfo with Galaxy II, bring the club to eight homegrown players.
The Galaxy kick off their 2017 campaign against FC Dallas on Saturday.