Gandhi who had contested the high-stake polls from two seats, won the election from Wayanad in Kerala, while lost the Gandhi bastion of Amethi to BJP’s Smriti Irani.
Irani defeated the Gandhi scion by a margin of over 55,000 votes.
Rahul took the oath in English.
Minutes before taking oath, he tweeted that his fourth consecutive term as a member of the Lok Sabha begins Monday.
"Representing Wayanad, Kerala, I begin my new innings in Parliament by taking my oath this afternoon, affirming that I will bear true faith and allegiance to the Constitution of India", he tweeted.
Gandhi's tweet came after murmurs grew loud in the Parliament questioning his absence in the morning. Gandhi clarified in his tweet that he would be taking the oath in the afternoon.
Irani had contested unsuccessfully from Amethi against Rahul Gandhi in 2014. However, this year she managed to seize Amethi considered as the pocket borough of Gandhi family for long, after a neck and neck fight against Gandhi.
Irani also took oath as member of the 17th Lok Sabha today morning. She received the longest applause while reading the oath.
As soon as she was called for oath taking, ruling BJP members, including Prime Minister Narendra Modi, Home Minister Amit Shah, other union ministers and MPs were seen enthusiastically thumping the desk for a long time.
After taking oath in Hindi, Irani greeted pro-tem Speaker Virendra Kumar and also oposition leaders, including Rahul Gandhi's mother and UPA chairperson Sonia Gandhi, who reciprocated the greetings with a namaste gesture.
Rahul Gandhi was not present in the House at that time.