"Happy belated birthday to this beautiful soul," gushed Spears, while sharing a never-before-seen undated photo of her and sister. In addition, singer Justin Timberlake is also featured in the sweet photo.
"I love you so much and d--- it I wish I was as smart as you at 30," raved the "Toxic" singer. "It's weird to say that I was basically your momma when you were younger and yes I'm still older but your soul is and always has been wise."
Spears, 39, continued praising her little sister by writing, "I'm truly and genuinely inspired by your path and the life you have created for your beautiful children and I'm so f------ proud to call someone as beautiful inside as you my sister."
"Not only are you a beautiful soul you're so D--- STRONG," the Grammy winner furthered, noting how Jamie Lynn Spears would always win their countless play wrestling matches.
"You put me to the floor in literally two seconds after I gave all my might," Spears remarked, adding, "I'll never forget it!!! Maybe we should have a rematch."
Noting how "things have changed since 6 years ago," Spears noted that her feelings for her little sister have always remained the same by closing out with a heartfelt "wish I was as strong as you and happy birthday."
Jamie Lynn Spears turned 30 on April 4.