The 21-year-old has made two substitute appearances for the Saints, including a Premier League debut in February 2015, but has not featured for them since.
He played four times in a month-long loan spell at Crewe in the 2015-16 campaign, scoring once.
"I'm really happy to be here and I'm excited by the challenge ahead," Seager told the club website.
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League One side MK Dons have signed Southampton striker Ryan Seager on a season-long loan deal.