Southport put three goals past Eastwood to claim the Blue Square North title and secure promotion to non-league's top flight.
Matty McGinn put the visitors ahead after just eight minutes but Eastwood held tight going into half-time.
Southport gained momentum in the second-half and two goals from Chris Simm and Chris Leaver sealed the win.
Title-rivals Fleetwood beat Stalybridge 2-0 but it was not enough to stop the Sandgrounders clinching promotion.
Southport manager Liam Watson told BBC Radio Merseyside: "We haven't got any superstars, we've just got the best group of lads I've ever had the pleasure of working with.
"It was all about winning the League, everyone today was just outstanding so it was never in doubt.
"This is my third Championship in five years. Everyone is special but with all the baggage, which came with this makes it the sweetest of them all."
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