Arnold Rothstein (played by Michael Stuhlbarg), affectionately nicknamed "The Big Bankroll", is the biggest, most powerful gangster in the country at the beginning of the series. Despite his status, he is known for his calm and polite demeanor as well as his fondness for gambling and picky food habits. He is based on the historical figure of the same name.
As a consummate gambler, strategic genius, and forward-thinking intellect, Rothstein never bets unless the fix is in. "The Big Bankroll" has incredible resources at his disposal, and will go down in history for his most famous, and possibly most impressive fix - the rigging of the 1919 baseball World Series. It is Rothstein's activity which brings Agent Nelson Van Alden to Atlantic City, and brings the heat on Nucky Thompson. After Nelson Van Alden investigates Nucky's operations, he diverts his attention to him, believing Nucky to be the "bigger fish" of the two.
- Main article: Arnold Rothstein Season 1
- Main article: Arnold Rothstein Season 2
- Main article: Arnold Rothstein Season 3
- Carolyn Rothstein: Wife
- Lucky Luciano: Protégé
- Meyer Lansky: Protégé
- Abe Attell: Lieutenant
- Davey Murdoch: Lieutenant (deceased)
- Simon: Employee, distant relative (deceased)
- "Sleepy Bill" Burns: Associate
- Nucky Thompson: Ally and business partner, boss of the Atlantic City Organization (deceased)
- Johnny Torrio: Ally, boss of the Chicago Outfit
- Waxey Gordon: Ally and business partner, Philadelphia crime boss
- Jimmy Darmody: Associate, former boss of the Atlantic City Organization (deceased)
- Big Jim Colosimo: Former boss of the Chicago Outfit (deceased)
- Joe Masseria: New York mafia boss (deceased)
- Frankie Yale: Associate and Brooklyn Mafia Boss (deceased)
- Al Capone: Associate through Torrio
- Eddie Kessler: Associate through Thompson (deceased)
- Owen Sleater: Associate through Thompson (deceased)
- Herman Kaufman: Associate through Gordon (deceased)
- Bill McCoy: Alcohol importer
- Chalky White: Associate through Thompson (deceased)
- Lolly Steinman: Casino manager in Atlantic City
- Sixtus D'Alessio: Ordered death, Given up to Nucky Thompson, killed by Al Capone
- Ignatius D'Alessio: Ordered death, Given up to Nucky Thompson, killed by Richard Harrow
- Leo D'Alessio: Ordered death, Given up to Nucky Thompson, killed by Jimmy Darmody
- Pius D'Alessio: Ordered death, Given up to Nucky Thompson, killed by Richard Harrow
- “There was a man once—I don’t recall his name—frequented the billiard parlors downtown. He made a comfortable living wagering whether he could swallow certain objects, billiard balls being his specialty. He’d pick a ball then take it down his gullet to here, and regurgitate it back up. And one evening, I decided to challenge this man to a wager: ten thousand in cash for him to do the trick with the billiard ball of my choosing. Now, he knew I’d seen him do this a dozen times, so I can only surmise that he thought I was stupid. We laid down the cash, and I handed him the cue ball. He swallowed it down, it lodged in his throat, and he choked to death on the spot. What I knew and he didn’t was the cue ball was one sixteenth of an inch larger than the other balls—just too large to swallow. Do you know what the moral of this tale is, Mr. Yale?" "The moral of the story is that if I cause a stranger to choke to death for my own amusement, what do you think I’ll do to you if you don’t tell me who ordered you to kill Colosimo?” ("The Ivory Tower")
- "This war ends here. Any bad blood or past transgressions of whatever nature, business or personal, are hereby nullified." (A Return to Normalcy)
- "A sound elimination is the basis of good health." ("What Does the Bee Do?")
- "Some things, Charlie, you have to swallow." ("What Does the Bee Do?")
- "I've made my living, Mr. Thompson, in large part as a gambler. Some days I make twenty bets, some days I make none. There are weeks, sometimes months, in fact, when I don't make any bet at all because there is simply no play. So I wait, plan, marshal my resources, and when I finally see an opportunity and there is a bet to make... I bet it all." ("Two Boats and a Lifeguard")
- "Nothing says 'I'm sorry' like money."
- "I have a crystal ball Charlie, you know that. I see....you're not wearing your trousers."
- "Mr. Darmody, don't you find it peculiar that neither one of us has mentioned Nucky Thompson spent last night in jail?"
- "Reputation takes a lifetime to build, but only seconds to destroy."
- "I prefer to make my living honestly."
- "There is actually. You can kill someone for me."
- "No you're not Charlie, you're with his mother."
- "Mr. Doyle, why am I talking to you?... But why am I calling you? Why is that even occurring? If I had my liquor on the timely basis your employer promised... if in fact I could reach your employer, who now seems to think acceptable business practice is to disappear for days on end, this conversation would not be taking place and that would make me... very happy. I want what I paid for. I want it now. And I don't ever want to find myself chatting with you again. Is that clear?" (Blue Bell Boy)
- A deal will always wait and a fool will always rush in (A man... a plan)
- ''All the mans troubles come from his inability to sit quietly in a room by himself."
- "Perhaps, get something better."
- "It's bad for business?"
- "And what would you know about it, conducting yours like a drunken shopkeeper?"
- "It's not alright. Do you think I entered into this arrangement because I value your companionship? You are a convenience, of geography and supply. You promised a quantity and a price. You have failed to deliver! And now, owing to your inability to manage your affairs in New Jersey - a state I have little interest in or affection for - you expect me to start a war? In New York? Where things actually matter?"
- "Because of your own cavalierness! Because you run off to Manhattan at a moment's notice to rut with some showgirl!"
- "Why? You can't. Do you even begin to understand how weak that makes you look?"
- "I practice discretion."
|Season one appearances|
|Boardwalk Empire||The Ivory Tower||Broadway Limited||Anastasia|
|Nights in Ballygran||Family Limitation||Home||Hold Me in Paradise|
|Belle Femme||The Emerald City||Paris Green||A Return to Normalcy|
|Season two appearances|
|21||Ourselves Alone||A Dangerous Maid||What Does the Bee Do?|
|Gimcrack & Bunkum||The Age of Reason||Peg of Old||Two Boats and a Lifeguard|
|Battle of the Century||Georgia Peaches||Under God's Power She Flourishes||To the Lost|
|Season three appearances|
|"Resolution"||"Spaghetti and Coffee"||"Bone for Tuna"||"Blue Bell Boy"|
|"You'd Be Surprised"||"Ging Gang Goolie"||"Sunday Best"||"The Pony"|
|"The Milkmaid's Lot"||"A Man, A Plan..."||"Two Imposters"||"Margate Sands"|
|Season four appearances|
|"New York Sour"||"Resignation"||"Acres of Diamonds"||"All In"|
|"Erlkonig"||"The North Star"||"William Wilson"||"The Old Ship of Zion"|
|"Marriage and Hunting"||"White Horse Pike"||"Havre De Grace"||"Farewell Daddy Blues"|