Winning the race in 2 minutes, 4.22 seconds, her time at the low-key meet fell almost ten seconds short of the national record she set when she won the global title in 1:55.45 in Berlin last August.
Semenya, though, was happy with her time after a such a long absence.
"To come and run a 2:04 is not easy, especially after what happened," Semenya said. "I was a little bit nervous because it has been a long time not competing."
"It's a new beginning," she added.
Semenya appeared nervous before the race on a hot and humid night at the humble Kimpinen Stadium, where the expectant crowd had increased by 1,000 compared to last year to 4,136.
Running in a purple top and black shorts, Semenya, who started in lane 4, was level with most of her competition before pulling away on the home straight to beat a weak field.
Her appearance was the curtain closer of the event in the sleepy town 135 miles (220 kilometers) east of the capital Helsinki, that normally receives little attention outside of the Nordic country.
She said that she enjoyed running in front of a supportive crowd and that she thinks she might get the same reaction wherever she races from now on.
Mari Jarvenpaa of Finland finished the race second in 2:04.71 and Olha Yekymenko of the Ukraine third in 2:05.
Coach Michael Seme said that he was happy to get Semenya's career back under way.
"This first comeback helped us alot because now we're feeling stronger and stronger, said Seme. "The two days training here helped us a lot."
Manager Jukka Harkonen said he thought that her performance had been "fantastic" given that "she put on her spikes for the first time three days ago."
Semenya is aiming to be in gold medal contention at October's Commonwealth Games in New Delhi, India.
Her next race is at the Lapinlahti Games on Sunday, before she heads back to South Africa to continue training.
Harkonen said he expects her to run two or three seconds faster at the Lapinlahti meet.
It was as an 18-year-old in Berlin that Semenya burst onto the scene when she won in her first appearance at a major final. Her dramatic improvement in times and muscular appearance led the International Association of Athletics Federations to order gender tests.
This sparked outrage in South Africa, where she had returned home a national hero after a stunning victory. Public officials rallied behind her and her lawyers entered into negotiations with the IAAF that lasted 10 months.
Semenya refused to criticize the IAAF and said she can run faster than she did in Berlin.
"Maybe it was good for me to rest after I ran my fastest time last year," Semenya said. "I'm still young, the muscles are still developing, so yeah, I can run faster than that."
Semenya was left off South Africa's team for the upcoming African championships after failing a fitness test the day after she was cleared by the IAAF to continue running as a woman.