Crystalline Form I of ibrutinib, processes for its preparation, pharmaceutical compositions comprising the new Form, and use of Form I of ibrutinib for treating or delaying diseases or disorders related to activity of Bruton's tyrosine kinase (BTK) proteins are disclosed. The novel Form was characterized by X-ray powder diffraction, differential scanning calorimetry, and other techniques. It can be readily prepared and is suitable for use in the preparation of solid dosage forms.