The Christian and Hollywood actor jokingly said abstaining from foods including meat, dairy, eggs and solid fats could affect his performance during promotional interviews from his latest movie.
In a video posted on Instagram, he said: "Check it out, it's 21 days of prayer and fasting. It's going to coincide, coincidentally, with the Lego Movie 2 junkets, so by the time you see me, I'll probably be hallucinating."
Described by its creators as a biblical fast "to deny food for a spiritual purpose", the Daniel Fast has proven particularly popular in the United States.
They expect "hundreds of thousands" of believers around the world to mark January with a fast of some kind.
One maker of Daniel Fast welcomed the publicity Chris Pratt's announcement has generated about the programme.
They said: "The great news for all of us in this community is that the news outlets are all linking to this site and quoting the truths about the fast from here.
"Some are referring to the fast as a diet, a common error when people don't understand spiritual fasting.
"But I'm thankful that God is getting the glory as He is raised up and many may be inspired to look to Him and Christ's amazing love."
Participants only eat foods grown from the seed; fruit, vegetables, whole grains, legumes (chick peas, lentils etc.), plant-based liquid oils, nuts and seeds.
Refined and processed foods, leavening agents, sweeteners, alcohol, tea, coffee and fizzy drinks are excluded. The only beverage allowed is water.
It is inspired by a three-week fast by Daniel after he received a revelation from God about a "great war".
Daniel 10:3 (NIV) says: "I ate no choice food; no meat or wine touched my lips; and I used no lotions at all until the three weeks were over."
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