The Hamas-run website Al Rai said the men and women were killed by firing squad and warned that "the same punishment will be imposed soon on others".
It suggested a link between the killing of the alleged informers and Israel's targeting of top Hamas leaders, saying "current circumstances forced us to take such decisions".
A Gaza security official said 11 suspected informants were killed in Gaza City police headquarters. The official said they had previously been sentenced by Gaza courts.
Al Rai said they were killed after the completion of "legal procedures", suggesting a hastily arranged hearing. It marks the third time since the outbreak of the Gaza war six weeks ago that Hamas has announced the killing of alleged collaborators.
In pinpointing the whereabouts of the Hamas commanders, it is likely Israel relied to some extent on local informers.
Israel has maintained a network of informers despite its withdrawal from Gaza in 2005, at times using blackmail or the lure of exit permits.
Meanwhile, Israel-Gaza fighting has continued for a third day since the collapse of Egyptian-led ceasefire talks earlier this week.
An Israeli airstrike on a Gaza farm killed two Palestinians, a Gaza health official said.
By mid-morning Thursday, Israel had launched about 20 airstrikes on Gaza, while Gaza militants fired at least 26 rockets at Israel.
The renewed exchanges have dashed hopes for a lasting truce after a month-long war that has already killed over 2,000 Palestinians. Earlier this week, Hamas rejected an Egyptian truce proposal under which Israel would gradually ease its blockade of Gaza, without giving specific commitments.
A quick resumption of indirect talks between Israel and Hamas in Cairo also seems unlikely.