The event, which takes place at Somerley Estate in Hampshire, is hosted by Holy Trinity Brompton church and sees thousands of Christians gather for a week of teaching, worship and fellowship.
Delegates at the festival reported seeing police on site as those camping were packing up to leave the event which finished on Tuesday.
In statement to Premier, Hampshire Police said: "We were called at 05:06am on Wednesday 1 August to a report of a 21 year-old woman being sexually assaulted at the Focus Festival which was being held at the Somerley Estate, north of Ringwood at some point between 1:30am and 2am.
"The victim is receiving support from specialist officers.
"A 54 year-old man who is not from Hampshire has been arrested on suspicion of rape. He has been bailed with conditions until 29 August pending further enquiries."
Holy Trinity Brompton has told Premier it is a police matter so won't be making any comment.
The Focus event has been running since 1992.
This year's speakers included Bishop of London Dame Sarah Mullally and US author and speaker RT Kendall.
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