The placing shares were sold at 155p per share. The shares represent around 5.2% of the company.
Millicom retains 76.5mln Helios Towers shares or 7.6% of the company and those shares are now subject to a 90-day lock-up.
Helios Towers will not receive any proceeds from the placing.
In London, Helios shares were down 21p or 11.9% changing hands at 154.4p.