The Los Angeles pro sports teams' joint effort to support the wildfire victims with "L.A. Strong" shirts is a massive success -- TMZ Sports has learned more than 30,000 items have sold so far -- with all proceeds going to help the cause.

The 17 local organizations teamed up with Fanatics to make merch with the "L.A. Strong" phrase on Friday ... as a tribute to those impacted by the devastating wildfires.

We're told the orders for the merch -- priced at $35 each -- are among the most popular on the site ... with a whole bunch selling over the past three days.

"Since launching, LA Strong shirts/hoodies account for 3 of the top 4 and 5 of the top 10 selling products across Fanatics network of sites," a spokesperson told us.

All the proceeds will go to the American Red Cross and the L.A. Fire Department Foundation.

Several L.A. teams have made individual donations to support their city, like the Lakers, who are hosting a donation drive at Crypto.com Arena ahead of their game against the San Antonio Spurs on Monday.

The Rams and Chargers worked with the NFL and other teams to collectively donate $5 million to the wildfire relief fund.

In total, the teams pledged a combined donation of $8 million to help those affected by and fighting the fires.