Andrew J. Sioson

ANDREW SIOSON advises clients on a broad range of regulatory, compliance, enforcement, and transactional matters involving securities markets, financial institutions, and emerging technologies. He is recognized by Chambers FinTech for USA Nationwide FinTech Legal: Blockchain & Cryptocurrencies (2025) and by Best Lawyers: Ones to Watch® in America for Securities Regulation (2026).

Andrew represents broker-dealers, trading platforms, and financial technology companies in complex regulatory and enforcement matters before the U.S. Securities and Exchange Commission (SEC), Financial Industry Regulatory Authority (FINRA), Federal Reserve Board (FRB), Public Company Accounting Oversight Board (PCAOB), and other state and federal regulators. His practice spans issues related to cryptocurrency, tokenized assets, artificial intelligence (AI), and blockchain/distributed ledger technology. He has significant experience advising financial institutions and technology firms on implementing AI-driven solutions within applicable securities, market conduct, and data governance frameworks.

Prior to joining Sidley, Andrew served as special counsel at the SEC’s Division of Trading and Markets, where he supervised equities and options exchanges, alternative trading systems (ATSs), broker-dealers, and other key securities market participants. His work covered SEC rulemakings on order handling disclosures, market structure reform, self-regulatory organization (SRO) rule changes, market data and National Market System (NMS) plan governance, exchange-traded fund (ETF) listing standards, and Dodd-Frank Act implementation. Before his time at the SEC, Andrew practiced corporate law in New York, focusing on private funds, finance, and mergers and acquisitions.

Andrew is a registered certified public accountant (CPA) and has held a Series 7 license.

Andrew’s financial services and digital asset practice includes:

• Advising broker-dealers on a variety of regulatory and enforcement issues, involving Reg. NMS, Reg. SCI, Reg. SHO, Reg ATS, Reg T, the Market Access Rule (Rule 15c3-5), the Market Data Infrastructure Rules, the Customer Protection Rule (Rule 15c3-3), best execution obligations, stock-lending, recordkeeping rules, and cross-border registration requirements.
• Representing broker-dealers in negotiating clearing, prime brokerage, securities lending, and other agreements related to broker-dealer operations.
• Advising market participants on establishing and operating ATSs, clearing agencies, transfer agents, and national securities exchanges — including those facilitating the trading of traditional securities and tokenized or digital assets.
• Advising a multinational financial institution on implementing artificial intelligence (AI) tools and workflows across its operations while ensuring compliance with applicable SEC, FINRA, and data governance requirements.
• Drafting comment letters on behalf of clients addressing proposed SEC and SRO rulemakings.
• Representing digital asset companies in enforcement and examination matters.
• Facilitating regulatory approvals with FINRA and the SEC for broker-dealers and digital asset companies engaging in regulated activities.

Andrew is a member of Sidley’s top-ranked Securities Enforcement and Regulatory practice, which has been ranked Band 1 for Nationwide Financial Services Regulation: Broker-Dealer (Compliance & Enforcement) by Chambers USA since 2013. The practice has also been repeatedly named “Compliance Practice Group of the Year” by Law360 and “Law Firm of the Year” for both Securities Regulation and Litigation — Regulatory Enforcement by U.S. News — Best Lawyers®(2024 and 2025).

Clients described Andrew as “the lawyer you call when you have a difficult project and you want to get the job done right the first time,” (2025) noting that “his responsiveness and updates put me at ease” (2025). Others praised that “Andrew has been my go-to for SEC-related strategy… he’s good at finding the balance between risk mitigation and business pragmatism” (2025).

Andrew’s representative matters include:

• Advising Sony Bank in relation to establishing a stablecoin business associated with Sony’s content operations.
• Represented Goldman Sachs & Co. LLC and provided regulatory advice in a collaborative initiative between the Goldman Sachs Group, Inc. and The Bank of New York Mellon Corporation to employ blockchain technology in relation to recordation of securities ownership.
• Represented Cboe Global Markets, Inc. in its acquisition of Eris Digital Holdings, LLC, an operator of a U.S.-based digital asset spot market, a regulated futures exchange, and a regulated clearing house.
• Negotiated node and technology agreements for strategic partnerships with blockchain technology companies.
• Represented issuers in global offerings of digital securities, stablecoins, digital assets, and asset-backed blockchain tokens.
• Advised Oasis Pro Markets, LLC related to U.S. regulatory requirements for its digital security alternative trading system (ATS).
• Advised INX Limited, a cryptocurrency exchange that went public on Ethereum in 2020 and issuer of the first registered security token, on broker-dealer and blockchain matters.
• Represented Brevan Howard, among others, in the launch of digital asset funds.
• Represented Paxos Trust Company, LLC in connection with its application to be the first SEC-registered clearing agency to use a blockchain technology platform to clear and settle transactions.
• Advised U.S.-cryptocurrency intermediaries and buy-side institutions with respect to digital prime brokerage offerings and lending programs.
• Advised on formation of broker-dealers and registration of alternative trading systems to transact or custody securities and digital assets.
• Negotiated trading and custody agreements for broker-dealers, banks, and other financial services companies.
• Advised client in obtaining regulatory approval to facilitate trading of security tokens or digitally enhanced securities.

• Honorary board member, Berkeley Law Foundation (formerly chief financial officer)