Together with its coalition partner, the Japan Innovation Party (JIP), the LDP was projected to win 310 seats, NHK said, giving them a two-thirds majority.
"We received [voter] backing for Prime Minister Sanae Takaichi's responsible, proactive fiscal policies and a strengthening of national defence capabilities," LDP secretary general Shunichi Suzuki told media.
Takaichi dissolved parliament in January and called snap elections, a gamble that she hoped would provide her and her struggling party with a stronger footing in parliament going into the new year.