The 'Hot and Fun' singer apparently proposed to the fashion designer and model last summer, according to WENN.
Williams and Lasichanh were spotted at the world premiere of The Dark Knight Rises on Monday (July 16), where Lasichanh was seen sporting a large diamond ring.
The couple have a 3-year-old son named Rocket together, and are notoriously private with their relationship.
Williams has yet to publicly comment on the engagement reports.
The 39-year-old musician recently previewed a new track produced with Usher called 'Twisted'.
Listen to 'Twisted' by Usher and Pharrell Williams below:
Photo gallery - 'The Dark Knight Rises' world premiere:
Copyright: WENN Dan Jackman