Real estate sources tell us the 'RHONY' star is listing her Upper East Side townhouse for $8.75 million. Sonja initially listed it way back in 2013 for $9.95 million amid her divorce from J.P. Morgan heir John A. Morgan.

The 4-bedroom home has seen a lot of changes since then because we're told Sonja has given it a complete makeover over the years.

She had an offer in 2017, but turned it down because her daughter needed stability at home amid the divorce -- she would frequently bring friends home from boarding school, so they still needed the space.

As we've reported, she listed the 5-story townhouse for the 7th time in 2017 for around $10 mil ... still, to no avail.

Someone's missing out ... all the bedrooms feature wood-burning fireplaces, and there's also an elevator, sauna, gym, and terrace -- all sitting in the ritzy Madison Ave and Central Park neighborhood.

In January 2020, Sonja had an $11.75 million cash offer, but the buyer ended up dropping out in February when the pandemic began. Then, for a period of time, she couldn't show the home because of COVID.

After the lockdown lifted in the city, she rented the home to a family from Spain, before renting it out to a new tenant, for 32k a month. We're told, the tenant recently moved out.

Now, Sonja is back at it ... determined to get the pad rented again, or sold for the asking price.