What Google says
In Google's own words: “Negative keywords let you exclude search terms from your campaigns and help you focus on only the keywords that matter to your customers.” (Google help center)
Match types, and the close-variant catch
Negatives come in broad, phrase, and exact match, but they behave differently from positive keywords in one crucial way: negative keywords do not match close variants. Exclude "flower" and your ad can still show for "flowers". So you have to add the plurals and synonyms you want blocked, not just the root term. (Google says negatives do account for casing changes and for misspellings where Google changes the search results, so misspellings are only partly covered.)
Where they matter most
Tight negatives are what keep a broad-match or Performance Max campaign from drifting into junk traffic, and what keep a competitor-conquest campaign off your own brand. Reviewing the search terms report and adding negatives is one of the highest-return maintenance jobs in any account. They are also what keeps the competitor Performance Max campaign in the three-campaign B2B setup pointed at the right traffic.
A worked example
Say you sell prescription eyeglasses and your broad-match keyword "glasses" picks up "wine glasses" in the search terms report: 120 clicks at $1.50, so $180 spent with zero sales. Adding "wine glasses" as a phrase-match negative stops that spend. Keep the close-variant catch in mind: a negative broad match of "running shoes" blocks "blue running shoes" but not "running shoe", so the singular needs its own entry.
Common questions
What are negative keywords in Google Ads?
Negative keywords are terms you exclude so your ads don't appear for searches containing them. They stop budget being spent on irrelevant or off-intent queries, and can be applied at ad group, campaign, or account level, and to Performance Max.
Do negative keywords match close variants?
No. Unlike positive keywords, negative keywords do not match close variants, so excluding "flower" will not block "flowers". You need to add plurals, misspellings, and synonyms explicitly if you want them excluded.
Can you add negative keywords to Performance Max?
Yes. You can add negative keywords to a specific Performance Max campaign, apply campaign-level negative keyword lists, or use account-level negatives, which apply to all Search and Shopping inventory including Performance Max. Performance Max negatives apply to Search and Shopping inventory only, not to YouTube or Display.
Should I use negative keywords to keep Performance Max off my own brand?
Google recommends brand exclusions instead, because they also block common misspellings, variants, and subsidiary brands. Google positions Performance Max negatives as a control for brand safety and clearly irrelevant terms, since over-restricting can cut off valuable traffic.
What is the difference between negative broad, phrase, and exact match?
Negative broad blocks a search that contains all your terms in any order, negative phrase blocks a search containing the terms in the same order, and negative exact blocks only the exact terms in the same order with no extra words. None of them expand to close variants.