Are shoes najis after walking outside?

No, not by default. Ordinary street dirt on the soles of your shoes is considered pure unless you know something impure is stuck there. If you did step in something najis, like dog mess or urine, you clean it off, and scholars even allow wiping shoes on clean ground in some views.

Pure until proven otherwise

A basic rule in Islamic law is that things are pure unless you know they are not. Streets, pavements and parks are full of dust, water and dirt, and Muslims are not expected to assume that every step landed in something impure. This is why you do not have to wash your shoes just because you walked outside. The same logic applies to mud. See Is mud from the street najis?

If you stepped on something najis

If you know your sole touched something impure, such as dog mess or urine, the shoe needs cleaning before you pray in it. The Prophet ﷺ gave an easy method: "When any one of you treads with his sandal upon an unclean place, the earth will render it purified" (Sunan Abi Dawud 385).

Scholars differ on how far this goes. Some say wiping the shoe on clean ground is enough, relying on this hadith. Others, like the Shafi'is, say anything that is truly najis must be washed with water. A simple, safe approach is to scrape off the dirt, wipe the sole on clean ground and then rinse it with water if you can.

The Prophet ﷺ and his sandals

Abu Sa'id al-Khudri reported that while the Prophet ﷺ was leading the prayer, he took off his sandals. Afterwards he explained that Jibril had told him there was filth on them, and he said: "When any of you comes to the mosque, he should see; if he finds filth on his sandals, he should wipe it off and pray in them" (Sunan Abi Dawud 650). So the instruction was to check, wipe and pray, not to treat all shoes as impure. Shoes are fine at prayer when they are clean. See Can I pray with my shoes on?

Long clothes and dirty places

A woman with a long hem asked Umm Salamah what to do about walking in dirty places. Umm Salamah answered that the Messenger of Allah ﷺ said: "What comes after it cleanses it" (Sunan Abi Dawud 383). This is another sign of how easy Islam makes everyday life.

Simple habits

Take your shoes off at the door of your home and keep your prayer area shoe-free.
Use a doormat and wipe your soles on it.
If your shoes are wet from rain, snow or puddles, that is not a problem by itself.
If you see dog mess or something obviously dirty on a sole, clean it before you step on your prayer mat.
In a mosque, follow the local custom and use the shoe racks.

If you are not sure whether something impure is on your shoes, you do not have to assume it is. See Can I pray with a stain on my clothes?

Key points

Street dirt on shoe soles is pure unless you know it is impure.
If you stepped in something najis, clean it; some scholars say wiping on clean ground is enough (Sunan Abi Dawud 385).
The Prophet ﷺ taught us to check our sandals, wipe off any filth and pray (Sunan Abi Dawud 650).
Do not spend your day worrying about what you may have stepped on.

Tip: Keep a pair of indoor slippers or slip-on shoes by the door, and keep outdoor shoes there. It keeps your home and prayer space clean without any special effort.