The public telescope at the summit of Mount Lofty in the Adelaide Hills has been stolen, in what is not the first theft of its kind at the tourist attraction.

Security guards noticed the $8,000 telescope was missing when they performed a morning patrol last week.

Last year, a similar telescope specially designed for people who use wheelchairs and mobility devices was taken.

It has not been returned or replaced.

The Department of Environment, Water and Natural Resources described the thefts as "disappointing".

"The thieves who took this telescope stole it from the whole community," the department's chief executive Sandy Pitcher said.

"We are trying to provide great experiences for visitors to South Australian icon sites like Mount Lofty, but selfish acts like this really make that difficult."

Ms Pitcher said security lighting could help to discourage such thefts, but that would destroy the ability to "enjoy the lights of Adelaide".

"Because of these thoughtless acts, we have been forced to suspend work on improvements at the site and seek quotes for CCTV instead," she said.

More than 350,000 people visit the summit, 15 kilometres east of Adelaide each year, according to National Parks South Australia.