Is it haram to use my neighbor's Wi-Fi without asking?

Yes, if they haven't given you permission. Your neighbor pays for that connection, so it's their property, and using it without consent is like using anything else of theirs without asking. The fix is simple: ask, and offer to share the cost. Open public Wi-Fi that's offered to everyone is a different matter.

Wi-Fi is someone's property

It's easy to think of Wi-Fi as "just in the air." But your neighbor pays every month for it, and every device that connects uses part of their speed and sometimes their data allowance. In Islam, you may only use someone's property with their consent.

يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ لَا تَأْكُلُوٓا۟ أَمْوَٰلَكُم بَيْنَكُم بِٱلْبَـٰطِلِ إِلَّآ أَن تَكُونَ تِجَـٰرَةً عَن تَرَاضٍ مِّنكُمْ "O you who have believed, do not consume one another's wealth unjustly but only [in lawful] business by mutual consent." Quran 4:29

The Prophet ﷺ said that your blood, your property and your honor are sacred to one another (Sahih al-Bukhari 67). A small thing is still someone else's thing.

"But it isn't locked"

An open home network isn't automatically an invitation. Your neighbor may simply not know how to set a password. And if they gave you the password once when you visited, that doesn't mean they agreed to you using it every day. When you're not sure what they'd say, the honest answer is to ask.

Your neighbor has rights

Islam gives neighbors a special status. The Prophet ﷺ said that Jibril kept advising him about the neighbor until he thought the neighbor would be made an heir (Sahih al-Bukhari 6014). Respecting what's theirs, even something as small as their internet, is part of being a good neighbor.

Just ask

Be upfront: "Would you mind if I used your Wi-Fi for a while?"
Offer to pay: sharing the monthly cost is fair and often welcome.
Accept a "no" gracefully: it's their right, and it keeps the relationship good.

Once they agree, using it is completely halal. It might even start a good friendship.

Public Wi-Fi is fine

Cafes, libraries, airports, trains and city hotspots offer Wi-Fi to the public on purpose. Using it is fine, as long as you follow their conditions, such as being a customer if they ask for that. For a similar question about sharing access, see Is it haram to share my Netflix password?

Key points

A neighbor's Wi-Fi is their paid property.
Using it without permission isn't allowed.
Ask, and offer to share the cost.
Public Wi-Fi offered to everyone is fine.

Tip: If you've been using a neighbor's Wi-Fi without asking, knock on their door with a small gift, explain, and offer to chip in from now on. Honesty usually turns an awkward moment into goodwill.