Pineda converted 72 seconds after the opening kickoff and added goals in the fifth and 29th minutes to give Guatemala its first goals and win at the tournament, ending the Dominican Republic's stay with a third straight loss and outscored 27-0.
Guatemala entered the match after a 13-0 loss to the United States and 5-0 loss to Mexico.
"The losses were painful," Pineda said. "Obviously its going to hurt, for any team to let up so many goals. But we knew we had another opportunity for the work that we have been doing and to be able to finish strong and show the type of team we have. Maybe the league isn't professional in Guatemala, but we're improving and keep going."
With both teams already eliminated from contention for CONCACAF's two berths in the London Olympics, the result finished Guatemala with three points and in third place in Group B, behind the United States and Mexico, who were to play later Tuesday for the group title.
Maria Monterroso added two and Ana Lucia Martinez another for Guatemala, which missed a chance to add to its total when Pineda sent a second-half penalty attempt well over the crossbar.
Both Guatemala and the Dominican Republic were two of the youngest teams in the tournament. Guatemala started five teenagers and the Dominicans four, including the youngest of the eight-team event - 15-year-old forward Anajaira Claudio.
With everyone but Claudio - who had played in the previous two matches as a substitute - starting their third match in 10 days, the Dominicans appeared fatigued and slow.
Guatemala jumped ahead when Idiana Perez served a ball down the middle to Pineda, who ran past Carmen Polanco, one-touched into the area and shot low past keeper Heidy Salazar from just inside the 18.
The Chapines created almost an exact copy for the second goal. Martinez took a ball from Perez and again served a ball down the middle to Pineda, who ran just inside the area before tallying her second within five minutes.
Monteroso added her first in the ninth, volleying a bounding cross from Marylin Rivera at the penalty spot.
Martinez capitalized on a loose ball for her goal in the 53rd. Guatemala's Amarilis Lozhaiza dribbled into Dominican defenders Denny Vargas and Ana Frias, resulting in the ball bounding away where Martinez raced into the area to settle it, round Salazar and tap in for the finish.
Monterroso completed the scoring in the 91st minute, driving in a loose ball after two earlier shots were blocked following a four-on-one break.
"Unfortunately the results just didn't go our way, but we knew we had to come here and show growth," Guatemala coach Raul Calderon saicd. "Today we looked happy, we looked content out there and we showed the growth and our ability."
Pineda volleyed a ball off the bar in the 42nd, Monterroso headed a corner off the bar in the 71st and Martinez shot off the near post in the 89th in three other near misses, while the closest the Dominicans came to scoring was a missed bicycle kick attempt by Betzaida Ubri in the 16th.
The Dominicans final chance came in the 88th when Yaquesi Nunez was able to break in alone from midfield, but 16-year-old Guatemalan keeper Mariandre Rodoas raced off her line to smother the ball.