1. to react to something too strongly to respond to something with too strong an emotion or with unnecessary or excessive action
I was furious and yelled at him. He told me I was overreacting and to "chillax."
— Ben Stein The world isn't coming to an end. You shouldn't overreact to the negative news.
— Adam Bryant Some of his animals had become apprehensive and twitchy, overreacting to sound or touch …
— Mark Caldwell But the military police overreacted, using tear gas, rubber bullets and excessive force in front of news cameras that spread the images countrywide.
— Grant Wahl … the poisonous surfeit of hormones produced by the overreacting immune system.
— Shannon Brownlee Sometimes companies in a little bit of trouble are the best investments because the market has overreacted.
— Orson C. Clay

2. to respond to something with an emotion that is too strong or an action that is unnecessary to react to something too strongly

3. to respond with an emotion that is too strong or an action that is extreme or unnecessary After being criticized, he overreacted and quit the team.

4. to react excessively or too strongly
the body's immune system overreacts to the endotoxin
— Tina Adler