Hilary Duff Pens Sullen Tribute to Late Hayden Panettiere

Hilary Duff is paying tribute to her fellow former teen sensation Hayden Panettiere ... days after Hayden's shocking death.

The singer and actress shared an emotional note on social media Tuesday, saying it was "tough" to hear about Hayden's suspected overdose.

Hilary described Hayden as "so hardworking and effervescent" with a "big, gorgeous smile" before recalling their late-night chats as teens that bonded them together.

She wrote, "I remember staying up late, chatting and gossiping, and connecting because we were part of such a small sliver of people who actually understood the strange lot in life we were navigating.. so young, with no real idea how to make sense of any of it."

Hilary went on, "I so deeply wish this wasn't the end for you. Sending a heart full of love to your family, the many people you impacted and everyone who loved you. rest easy."

While Hilary and Hayden never had a project together, they were both major child and teen stars in the 2000s and shared an overlapping social circle.

As we reported, Hayden died Sunday at 36 after going into cardiac arrest. She was in a South Carolina apartment with her boyfriend, Brian Hickerson, when she was found unresponsive.

Dispatch audio mentions a possible "overdose" ... and we learned Narcan was found in the room.

TMZ revealed she and Brian had been seeing each other in secret before her death ... keeping things hush-hush due to their tumultuous past, which included Brian's arrest for domestic violence. Just last week, he asked a judge to reduce his past felony domestic violence charges to misdemeanors -- and expunge them -- but he was denied.