I realize the boy section is very long so feel free to do it in multiple posts and/or separate from the girls. J is a popular letter.
Girls
Jacqueline Dena or Jada Elly
Jade Marie or Jade Sheena Jezebel
Jade Yeshe or Jaeden Arie
Jagger Snow or James Olivia
Jamison Bess or Jana Marie
Jane Erin or Jasmine Ines
Jasmine Lia or Jasmine Mickael
Jasmine Page or Java Kumala
Jayde Carter or Jaz Elle
Jazmin Grace or Jenna Welch
Jennifer Danielle or Jennifer Jean
Jennifer Katharine or Jessa Lauren
Jessica Brooke or Jessica Marie
Jessica Rae or Jessica Starshine
Jessie James or Jessie Phoenix
Jett Ling or Jewell Kalaih
Jill Michelle or Jinger Nicole
Joanna Marie or Jocelyn Carlyle
Johannah Faith or Johnnie Rose
Jolie Ivory Imani or Joni Mason
Jordan Catherine or Jordan Kay
Jordyn-Grace Makiya or Jordana Nicole
Jorja Bleu or Joscelyn Skye
Josephine Kate or Josephine Sophia Ivalo Mathilda
Josie Brooklyn or Josie James
Josselyn Sydney or Judah Victoria
Julia Mimi Bella or Julianna Tex
Julie Marie or Juliet Leigh
Julitta Dee or Juna Meredith
June Catherine or June Joanne
Junia Rosa Ruth or Justice Jay
Boys
Jaafar Jeremiah or Jace Vahn
Jack Adam or Jack Alexander
Jack Antonio or Jack Breckenridge
Jack Clark or Jack Douglas
Jack Haydon or Jack Henry
Jack Holden or Jack Joseph
Jack Leon or Jack Lion
Jack Matthew or Jack Montgomery
Jack Nicholas or Jack Oscar
Jack Paris or Jack Phillip
Jack Scott or Jack Steven
Jackson Blue or Jackson James
Jackson Kayse or Jackson Levi
Jackson Oliver or Jackson Phillip Deveraux Montgomery
Jackson Riley or Jackson Robert
Jackson Slade or Jackson William
Jackson Wright or Jacob Daniel
Jacob Emerson or Jacob Sutton
Jacques Honoré Rainier or Jaden Christopher
Jaden Christopher Syre or Jaden Gil
Jaden Rayne or Jagger Donovan
Jagger Jonathan or Jagger Joseph Blue
Jagger Michael or Jai Blue
Jaid Thomas or Jaimy Jermaine
Jake Stone or Jakob Danger
Jakob Dylan or Jakob Nolan
James Andrew or James Arthur
James Hamilton or James Haven
James Hoke or James Hunter
James Jeffrey or James Joseph
James Knight or James Leroy Augustin
James Louis or James Monroe
James Padraig or James Roderick
James Smallwood or James Thomas
James Timothy or James Wilkie
Jameson Moon or Jameson West
Jared James or Jason Michael
Jasper Breckinridge or Jasper Michael Brown
Jasper Warren or Jathan James
Jax Copeland or Jax Joseph
Jaxon Creed or Jaxon Wyatt
Jay Phillip or Jayden James
Jedediah Lindsay or Jedidiah Robert
Jeffrey Charles or Jeffrey Michael
Jelani Asar or Jeremiah Robert
Jeremy James or Jeremy Maldonado
Jermaine La Juane or Jermajesty Jermaine
Jesse Gregory or Jesse Jameson
Jesse Mojo or Jesse Wayne
Jett Barker or Jett Hamilton
Jett Riviera or Jimmy Jalen
Joaquin Antonio or Joe Alfie Winslet
Joe Tedrick or Joel Kevin
Johan Riley Fyodor Taiwo or John Abraham
John Basil or John Charles Julian
John Christopher or John David
John Edward Thomas or John Henry
John Ira Shepherd or John Joseph
John Nash or John Owen
John Paul or John Phillip
John Rocco or Johnny Christopher King
Joming Jaycee or Jonah Christopher
Jonathan Rosebanks or Jonathan William
Jones McCoy or Jordan Ezra
Jordan Maxwell or Jordan Zain
Joseph Charles or Joseph Frederick
Joseph Garrett or Joseph Gunnar
Joseph Marciano or Joseph Michael
Joseph Stockton or Joseph Washington
Josey Hollis or Joshua Bishop
Joshua Davis or Joshua James
Josiah Matthew or Jourdynn Michael
Journey Jameson or Journey Jette
Journey River or Judah Miro
Jude Gordon or Jude Malcolm
Jules Andrew or Julian Franco
Julian Fuego or Julian Henry
Julian Kal or Julian Murray
Julius Robert Bay
Julius Setta or Junior Savva Andreas
Justin Dior or Justin McKenzie Phillip
Justin Ryan or Justin Samuel