The event runs from Monday, August 31, until Sunday, November 29, following a spring survey which proved popular during lockdown.
The survey, which usually only takes place once a year, asks people to record the presence of mammals in their gardens or local green spaces online.
PTES saw record breaking numbers of volunteers taking part earlier this year.
The charity wants to encourage those with a newfound love for wildlife-watching to join those who submit records every year and take part this autumn.
Volunteers across the UK are asked to spend a little time each week looking out for wild mammals and record sightings at www.ptes.org/LwM.
Comparing the results with those from earlier surveys will enable conservationists to better understand how wild mammal populations nationwide are changing.
David Wembridge, PTES mammal surveys co-ordinator, said: “More than 1100 people took part in our spring survey and over 10,000 mammal records were submitted – the highest on record since the survey began 18 years ago.
“’We don’t want to lose this momentum and want to keep sight of the connection with nature lockdown afforded us.
“By taking part again this autumn, we can gain a unique insight into the lives of our wild neighbours.”
In the spring, the top five species in urban areas were hedgehogs, red and grey squirrels, foxes, bats and badgers.
For more information on the charity’s work across the UK, visit ptes.org.