The deal, the mobile network's most expensive ever, lasts for two years and includes unlimited data and talk time.
It is for the largest version of the device, and the same contract with an iPhone 8 costs £44 per month, plus a £99 upfront fee.
The handset is sold by Apple for £1,149 - but Three's deal could end up costing the consumer over £2,100.
The company said it has different contracts to fit different consumers.
"There are a small amount of people that prefer not to pay an upfront cost yet want access to all-you-can eat data, minutes and texts and this is aimed at them," Three told the BBC.
Other companies are offering deals between £80 and £88, but customers have to pay an upfront fee of around £100.
O2 is charging £86 per month plus a one-off £119.99 for the same unlimited deal on the larger phone, while Vodafone does not offer unlimited data, but its most expensive tariff is £80 per month plus an upfront payment of £100.
The much-anticipated iPhone X is currently pre-order only.