The Russian Soyuz spacecraft carrying Koch parachuted down to the grasslands of Kazakhstan at around 09:12 GMT.
She spent 328 days on the International Space Station (ISS), surpassing the previous record held by fellow American Peggy Whitson.
Her stay is just 12 days short of the all-time US record set by Scott Kelly, who was on the ISS from 2015-2016.
"I'm so overwhelmed and happy right now," she told reporters as she sat outside the capsule, shortly after it touched down in the snow.
Ms Koch surpassed the 289-day record set by fellow American Ms Whitson on 28 December last year. But her return to Earth sets the marker for future space travellers to beat.
Whitson still holds the record for most time spent in space by a woman, accrued over the course of three spaceflights from 2002-2017.
During her mission, Koch completed 5,248 orbits of the Earth and travelled 223 million km (139 million miles) - the equivalent of 291 round trips to the Moon from Earth.
She returned on the Soyuz with two other crew members - the Italian European Space Agency astronaut Luca Parmitano and Russian cosmonaut Aleksandr Skvortsov. They touched down near Dzhezkazgan in central Kazakhstan.
"For me, it's all about the honour I feel to follow in the footsteps of my heroes," Christina Koch told journalists on Tuesday during a live link-up from the ISS. She added that she wanted to inspire the next generation of space explorers.
"For me, it was important to see people that I saw a reflection of myself in, growing up, when I was envisioning what I could do with my life and what my dreams might be. To maybe be that source of inspiration for someone else is just such an honour," she said.
Ms Koch was involved in another spaceflight milestone during her stay of nearly 11 months on the ISS. On 18 October last year, she undertook the first all-female spacewalk alongside her compatriot Jessica Meir.
The pair spent seven hours outside the ISS replacing a failed power control unit.
Recalling the historic event with Meir, Ms Koch told NBC News: "When we first got the 'go' to come out of the airlock, and we ended up coming out, we were holding on to a handrail and we just caught each other's eyes.
"We knew how special that moment was and I'll never forget that."
