They've been used to brighten up people's areas, and spread a message of hope.
Recently some people have also put a picture of a yellow heart in their windows - they have a special and personal meaning for those who put them up.
Here's everything you need to know.
The yellow hearts are being used to remember loved ones.
The idea was started by Becky and Hannah Gompertz and their granddad, David, after their grandma, Sheila, passed away.
They wanted a special way to remember her and others who have died because of coronavirus.
David also wanted a way for people to show other people that they are grieving, so they don't feel alone.
They came up with the idea of putting a picture of a yellow heart in their window, and shared it on social media.
So far, over 2000 people have joined their group online and lots of people have copied the idea.
Becky said: "It just shows that this is something that can really help support people, and we just hope that it will do some good...it's good that people are able to grieve together."