The woman, who appears to be dressed all in black, was spotted by police monitoring the motorways CCTV at around 7.25am.
North West Motorway Police said she was seen running across both the northbound and southbound carriageways before traffic was brought to a halt.
The force reported the woman to Greater Manchester Police (GMP) and Highways England, who rushed to the scene to stop traffic.
GMP said it was also "called several times" by motorists who were concerned about the woman's erratic behaviour.
A police spokesman said: "Officers were called at about 7.25am today (August 2) over concerns for a woman walking south on the northbound M6 between junctions 26 and 25.
"A woman in her 20s was detained by police a short time later."
Motorists were temporarily held both ways between J26 (Standish, Parbold ) and J25 (Bryn) before lanes were reopened at 8.35am.
It had led to northbound congestion back to J23 (Haydock) whilst traffic on the southbound carriageway is queueing back to J27 (Standish).
This morning, police initially reported that the person on the carriageway was believed to be a man, with the force tweeting: "There is a pedestrian on the M6 J26 to J27 Northbound. We are monitoring on CCTV and Police & Highways have been deployed.
"The male was running across both the northbound and Southbound c/way - Live lane. The traffic needed to be stopped for not just his safety but for road users."
But after police arrived at the scene, officers established that it was a woman walking in the lanes. She was picked up by Merseyside Police, who patrol that stretch of the M6.
The woman has been escorted to safety and taken into police custody due to concerns for her welfare.