Is it rude to leave someone on read as a Muslim?

No, leaving someone on read is not a sin by itself, but making a habit of ignoring people can be unkind. You don't owe instant replies, and Islam values guarding your time. Still, replying to greetings, important messages, and people who depend on you is good manners, and returning salam is a duty.

You're not obliged to reply instantly

Phones make everyone reachable all the time, but Islam doesn't require you to be. You may be praying, working, driving, or simply resting. Leaving a message for later, or not answering a random group chat, isn't haram. Protecting your attention can even be part of good worship. See How do I deal with social media addiction as a Muslim?

Returning salam is a duty

If someone sends "Assalamu alaikum", replying is an obligation, and the Quran encourages replying even better:

وَإِذَا حُيِّيتُم بِتَحِيَّةٍ فَحَيُّوا۟ بِأَحْسَنَ مِنْهَآ أَوْ رُدُّوهَآ "And when you are greeted with a greeting, greet [in return] with one better than it or [at least] return it..." Quran 4:86

Many scholars say a written salam should also be answered, at least in your heart and words when you read it, and ideally in your reply. The Prophet ﷺ listed returning the greeting among the rights one Muslim has over another (Sahih al-Bukhari 1240). See Can I say "Assalamu alaikum" if I'm not Muslim?

When ignoring becomes a problem

Family: regularly ignoring parents' messages can hurt them and touches on the duty to honour them.
Commitments: if you agreed to something at work or with friends, silence can mean breaking a promise.
Grudges: deliberately ignoring a Muslim for more than three days because you're angry is forbidden.
Someone in need: a friend who's struggling deserves at least a short "I saw this, I'll call later".

When not replying is fine or even better

Flirty messages from the opposite gender you're trying to step away from.
Trolls, scams, and people trying to start arguments.
Late-night chats that would push you past Fajr.

Good digital manners

A quick reaction or "Got it, will reply soon" costs nothing and shows respect. If read receipts make you feel pressured, it's fine to turn them off. Kindness in small things is part of a Muslim's character. Even a smile is charity, and so is a kind word.

Key points

Leaving someone on read isn't a sin in itself.
Returning a salam greeting is a duty.
Ignoring someone out of a grudge for over three days is forbidden.
A short acknowledgement is kind and easy.

Tip: Set aside two short "reply windows" a day, like after Dhuhr and after Maghrib. You stay reachable without being glued to your phone.