The 26-year-old centre-back, who had a spell on loan at Roots Hall in 2013, has signed a three year-contract with an option for a fourth season.
"It's been a little bit of a long process but it's good to get it over the line and I'm looking forward to getting going," said Kiernan.
"I'm glad it's all sorted and I can get ready for the season ahead."
Kiernan played 13 games for Southend in the 2013-14 season, but his loan was cut short because of injuries at parent club Wigan Athletic.
The defender, who had a loan spell at Birmingham City in 2015, played 76 games for Rangers over two years.
He becomes the second Southend signing in as many days following the addition of forward Amadou Ba on Wednesday.
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Southend have signed defender Rob Kiernan from Scottish Premiership side Rangers for an undisclosed fee.