William Christopher Farrell was handed a three month suspended prison sentence for two offences of sexual assault at Nesscliffe Training Camp, near Oswestry, in May 2016.
At Shrewsbury Crown Court, Judge Peter Barrie said Farrell's actions could potentially put teenagers off joining the cadets in the future.
The 61-year-old, of Church Green, Cockshutt, near Ellesmere, was found guilty of two charges following a four-day trial last month.
Brendan Reedy, mitigating, said: "He appears to appreciate what he's done and understands that his behaviour was unacceptable."
Farrell kissed the victim on the cheek shortly after she arrived at the training camp for a weekend residential course.
The following day he touched her lower back and bottom during a training exercise.
Judge Barrie said: "It has been suggested that I should not treat this as particularly serious because it involved a social kiss when the cadets arrived and rubbing her bottom on the pretence that you were trying to brush leaves off her.
"But, your behaviour caused her real and serious distress. I think it's wrong to dismiss this kind of behaviour."
He said Farrell's actions were a "serious breach of trust".
"You have brought the army cadets into disrepute and could have put off young people who could benefit from the training," he added. "However, I think you have learnt your lesson."
The sentence was suspended for 12 months and Farrell was ordered to pay £200 compensation to the victim, told to complete 30 hours rehabilitation, 100 hours of unpaid work and was made the subject of a sexual harm prevention order.
His behaviour came to light when another cadet reported her concerns to officials.