The rally will be addressed by Rashtriya Swayamsevak Sangh chief Mohan Bhagwat, he said.
The Assam unit of the RSS includes some districts of Assam, the entire Nagaland and Meghalaya.
The 'Luitporia Hindu Samabesh' is being organised ahead of the February 27 Assembly elections in Nagaland and Meghalaya.
The BJP is aiming to make further inroads into the northeast after having formed governments in Assam and Arunachal Pradesh.
"A mega public rally of the RSS's Assam unit will be organised on January 21 in Guwahati. The meeting is expected to be attended by more than one lakh people, including over 33,000 Sangh workers," the Sangh's Assam prachar pramukh (publicity incharge) Shankar Das told PTI.
Das said over 20,000 families from across the three states have decided to provide five food packets each for distribution during the rally.