When what actually happens is the opposite of what everyone - characters and readers alike - reasonably expected.
Situational irony occurs when the outcome of a situation is fundamentally different from what was anticipated. The key word is 'fundamentally' - it's not just unexpected, it's the opposite of what logic suggested would happen. A marriage counselor getting divorced, a locksmith locked out of their own house, a survival expert getting lost in the woods. The gap between reasonable expectation and actual result is where situational irony lives.
Situational irony powers some of the most memorable moments in fiction - twist endings, shocking reveals, and those gut-punch scenes that reframe everything the reader thought they knew. It also reflects something true about life: things rarely go the way we plan. Using it well makes your stories feel both surprising and honest at the same time.
She sells her hair to buy a chain for his watch; he sells his watch to buy combs for her hair. Each sacrifice makes the other's gift useless - the perfect double situational irony.
Winston's belief that he's found allies in the resistance turns out to be a trap set by the very system he's fighting. His rebellion was monitored and managed from the start.
Dorothy spends the entire journey trying to reach the Wizard so she can go home, only to discover she had the power to go home all along.
Getting rained on isn't ironic. Getting rained on while carrying the umbrella you bought specifically for today's forecast - and left closed because the morning was sunny - is getting closer.
Good situational irony should feel like it grew naturally from the story's logic. If the twist feels arbitrary, readers won't find it ironic - they'll find it annoying.
When the ironic outcome hits, trust the reader to feel it. Don't have a character say 'Isn't it ironic that...' - that's the literary equivalent of explaining a joke.
Write a flash fiction piece (under 500 words) where a character takes a specific action to prevent something, and that exact action causes the thing they feared. Plan the ending first, then work backward to set up the expectation. Read it back and check: does the ending feel both surprising and inevitable?