Performance Max gives you very few levers over where it shows up. Negative keywords and brand exclusions say where not to go. Search themes are the opposite lever: a way to tell Google what your customers are searching for, in your own words, so the model starts from your language instead of guessing it from your landing page. Used well, they are a cheap way to improve relevance. Used carelessly, they are a list of keywords people mistakenly expect to behave like keywords.
What a search theme is
A search theme is a short phrase you add to an asset group to describe the searches and topics you want the campaign to learn from. It is a signal, not a trigger. Unlike a Search campaign keyword, a theme does not guarantee an impression for a matching query, and it does not limit the campaign to queries that match. It supplements Google's automated targeting; it does not replace it.
The exact steps
In the current interface: open Asset groups in the campaign menu, switch to the Summary view, select the pencil icon next to Signals on the asset group card, and choose Search themes. Type each theme and save. Themes live at the asset group level, so each asset group carries its own list, which is the reason to keep asset groups organized by product line or audience rather than lumping everything together.
The limits
Each asset group accepts up to 50 unique search themes, and each theme must be under 80 characters. That ceiling has moved: the feature launched with 10 themes, rose to 25 in late 2023, and now sits at 50. Treat the number as a ceiling rather than a target. Twenty precise themes usually teach the model more than fifty overlapping ones.
Search themes versus audience signals
Both live under the same Signals card, and both are guidance rather than restriction, but they answer different questions. Audience signals describe who converts: your customer lists, custom segments, and demographics. Search themes describe what they are looking for. The practical rule is to use both: audience signals to say what a good customer looks like, search themes to say what that customer types. Neither fences the campaign in. The algorithm is free to go beyond both.
How to write themes that help
Write them the way a buyer would phrase the problem, not the way your product page names the feature. A B2B software team might use "automate lead routing" or "round robin lead assignment" rather than a product name. Keep each theme to one idea, cover the distinct use cases each asset group serves, and avoid near-duplicates. Check the search terms insights after a few weeks: if the queries you get look nothing like your themes, the themes are not the thing steering the campaign, and the asset group's content and landing page deserve a look.
What search themes don't override
Themes sit inside your exclusions, not above them. Brand exclusions and negative keywords still apply, so a theme that overlaps with an excluded brand or term will not make those queries serve. That is the right way to think about the whole setup: exclusions are the walls, and themes are the direction you point the campaign inside them. If you have not yet set up the walls, start with adding negative keywords to a Performance Max campaign and excluding brand searches.
Make it a review habit
Add themes when you launch an asset group, then revisit them with the same cadence you use for negatives. Remove themes that pull irrelevant queries, add the phrases that appear in converting search terms, and note each change in a log so a performance shift can be traced to what you edited. Search themes are a small control, but they are one of the few ways to put your customers' own language into an automated campaign.
Sources: Google Ads Help, "Use search themes with your Performance Max campaign"; PPC Land on the search theme limit increase; Analyzify, How to Set Up PMax Search Themes; WordStream, Search Themes for Performance Max.