STAG Industrial shares rose 7% to $35.86 in after-hours trading Monday as the real-estate investment trust was selected to join the S&P MidCap 400.
S&P Dow Jones Indices said the addition is effective prior to the open of trading on Thursday.
STAG replaces public safety technology company Axon Enterprise, which is joining the S&P 500 as First Republic Bank will be removed. First Republic was seized early Monday by the Federal Deposit Insurance Corp.
Axon shares were down 0.5% to $215.01 after hours.
Write to Josh Beckerman at josh.beckerman@wsj.com