Thailand dominated from the start, attacking in waves and outflanking the Indonesian defenders on the National Stadium pitch. Rungrat got the War Elephants on the scoresheet with a decisive strike in the 11th minute and midfielder Thitiphan Puangjan followed up at the half-hour mark with a header in heavy traffic.
Rungrat struck again seven minutes after the break, and defender Narubadin Weerawatnodom pounced on a stray ball just outside the area to drive home Thailand's fourth goal in the 57th minute. Midfielder Chanathip Songkrasin added a fifth goal after a scramble on the edge of the box a minute before full time.
The victory sets up an enticing final against Myanmar, the surprise package of the tournament, who defeated Vietnam 2-1 earlier in the day. The gold-medal match will take place at 7.30pm Thailand time on Monday.
Myanmar has lost twice to Thailand in its only appearances in the SEA Games finals — in 2007 and in 1993, the last time the games were held in Singapore — since the golden run of Burma ended with a fifth straight title in 1973.
The defeat was a bitter one for Indonesia, who had been hoping to secure some glory before beginning a spell in the football wilderness.
Fifa earlier this month suspended the country from international competition on grounds of government interference in the national football association. However, it allowed the Under-23 side to see out its SEA Games campaign before the suspension took effect.
In the first semifinal on Saturday, Myanmar showed impressive energy and commitment as they overcame Vietnam in their quest for their first SEA Games football gold in 42 years.
Little was expected from the under-23 side who were missing key players held back for the start of the World Cup qualifying campaign and also competing at the under-20 World Cup in New Zealand but their fighting spirit has captivated their countrymen and many neutral fans as well.
Adopting the slogan "No Stars, No Problem" they went unbeaten through the group stages with impressive victories over Indonesia and hosts Singapore, but faced their toughest task on Saturday.
They soaked up a lot of pressure before wide midfielder Aung Si Thu calmly stroked home a 39th minute penalty after Vietnam defender Ho Ngoc Thang stuck his elbow out to block a free-kick.
Vietnam continued to press before Vo Huy Toan finally levelled with 20 minutes to go after the ball kindly fell into his path in front of an unguarded net.
But plucky Myanmar got the lead back in the 78th minute when captain Tun Nay Lin's long-range effort looped off defender Nguyen Than Hien into the net.
Myanmar coach Lwin Kyi said after the match that his side was peaking at the right end of the tournament.
"No matter who we play ... we are more confident for the final," he said. "Not only the coaching group, but all the players."
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