Can I make dua for someone without telling them?

Why a secret dua is so special

The Prophet ﷺ said: "There is no believing servant who supplicates for his brother behind his back (in his absence) that the Angels do not say: The same be for you too" (Sahih Muslim 2732). Another narration in the same place adds that this dua "is responded" and that an angel says "Amen."

So when you quietly ask Allah to heal your friend, find your sister a good job or ease your coworker's divorce, you're doing two things at once: helping them and earning the same good for yourself. It's also one of the purest kinds of worship, because nobody sees it and there's nothing to show off. See What dua can I make for my parents?

Who you can pray for

• Friends and family: by name, for anything good in this life and the next.
• People you've never met: the sick on the news, refugees, people in a disaster, the whole ummah.
• People who hurt you: praying for their guidance is hard, but it often softens your own heart.
• Muslims who have died: the believers ask, "Our Lord, forgive us and our brothers who preceded us in faith."

Non-Muslim friends and family

Yes, you can pray for them without telling them too. Ask Allah to guide them, protect them, heal them and bless their lives. When Abu Hurairah's mother refused Islam, the Prophet ﷺ prayed, "O Allah, set the mother of Abu Huraira on the right path" (Sahih Muslim 2491). For the details, see Can I make dua for my non-Muslim family?

Should you ever tell them?

There's no rule either way. Keeping it private protects your sincerity. But sometimes saying "I've been praying for you" really lifts someone during an illness or a hard time, and that's a good deed too. Use your judgment: tell them if it will comfort them, keep it to yourself if it might feel like pressure or showing off. Avoid announcing it on social media just to look pious. See Should I give charity secretly or openly?