The 25 to 145-centimeter-high trees were discovered covering 800 of 4,630 square meters cultivated by Tran Hung, 38, in Chu Se District on Thursday afternoon.
Police also found a nursery where baby cannabis trees were being reared.
Hung said the nursery is owned by his grandmother and that he was assigned to manage it two years ago.
In late 2019, an acquaintance gave him cannabis seeds to grow as cattle feed, he said.
Police, having seized the entire cannabis crop, plan to expand their investigation.
Cannabis is on the list of narcotics forbidden in Vietnam.
Those growing cannabis on a small scale could be fined VND2-5 million ($86.16-215.40). Farmers found cultivating 500 to 3,000 plants could face jail terms of six months to three years, while those operating on a bigger scale could spend up to seven years in jail.