Steven Bonnar, the MP for Coatbridge, Chryston and Bellshill, crossed the fore and middle fingers as he held up his right hand and made an affirmation - a legal requirement to become an MP.
He later tweeted a screengrab from the Parliamentary video feed from the Commons chamber, in which his had was cropped out of the image.
"That's us all official now," Mr Bonnar tweeted. "I have been sworn in as the MP for Coatbridge Chryston and Bellshill. I look forward to serving all constituents to the best of my ability for the time Scotland remains within the UK."