River Markets

River Markets is a prime brokerage and execution terminal for prediction-market trading, built for hedge funds, proprietary trading firms and professional traders rather than casual bettors. It sits at #72 on Predicts.guru's builder leaderboard by all-time attributed volume, having routed $6.2M through Polymarket since its first attributed fill on 2026-04-28, with 140 of live history behind it.

The terminal itself runs in the browser rather than a native app, presenting a multi-book blotter that aggregates order books from every connected venue, including Polymarket, Polymarket US and Kalshi, with Novig listed as coming soon. A trader places, edits and cancels orders across all of them from one screen instead of switching tabs, and positions, fills and profit and loss consolidate into a single view regardless of which exchange filled the order.

A trader signs up at River Markets' own dashboard, generates an Ed25519 API key pair there, and either trades through the browser terminal or scripts orders with River's Python SDK. The private key signs every request locally and never leaves the trader's machine; River's server stores only the matching public key.

Exchange access is organized into subaccounts, each holding its own encrypted credentials for a connected venue such as Polymarket. Orders reference a single River ID rather than an exchange-specific ticker, and River's smart order router matches the same contract across venues and sends the order to whichever connected book is quoting the best price at that moment, splitting fills where that improves the average price. Complex order types, icebergs, pegs, take-profit and stop-loss, are managed by River rather than by the venue itself, so cancelling the parent order unwinds every child order at once.

Positions and balances stay in the underlying Polymarket or Kalshi account that a subaccount connects to, and deleting a subaccount deletes its stored credentials along with it. What River Markets holds instead is the encrypted key material needed to place and manage orders on a trader's behalf, not the funds themselves.

• Multi-book order blotter

  Work order books across every connected market from one screen instead of tabbing between exchanges, useful once a trader is watching more than a handful of positions.

• Native execution algorithms

  Icebergs, pegs, take-profit and stop-loss orders are managed by River rather than the venue, so cancelling the parent order unwinds every child leg together.

• Cross-venue smart routing

  Matches the same contract across connected exchanges and splits an order to chase the best available price, which matters most for size large enough to move a single book.

• Subaccount strategy isolation

  Separate strategies, clients or capital into distinct containers, each with its own credentials and its own profit and loss attribution.

• Programmatic order placement

  A Python SDK signs requests locally with a key that never leaves the trader's machine, letting a desk script order flow instead of clicking through a browser.

Multi-venue proprietary trading desks

Firms already trading Kalshi or Polymarket US benefit most from one blotter and one profit and loss view instead of separate logins per exchange.

Algorithmic prediction-market traders

Locally-signed keys and a Python SDK suit traders who want to script iceberg or peg orders rather than click through a browser terminal.

Funds running multiple strategies

Subaccounts let a fund isolate books or clients without opening a new exchange account and a new login for each one.

Traders chasing category-specific flow

Politics drew the largest share of this builder's routed volume over the last thirty days, led by Will the Fed increase interest rates by 50+ bps after the September 2026 meeting.

Traders wary of blind custody

Locally-signed API keys and encrypted subaccount credentials suit someone who wants to limit what a compromised server could do with their exchange access.

River Markets' own site and documentation publish no fee schedule for order routing, execution or SDK access; the operator does not state a rate for using the terminal or a connected subaccount. Polymarket's own trading fee is set per market and applies the same way no matter which builder routes the order, so it is not something River sets and not a basis for comparing it with other tools. A trader should ask River directly what a subaccount costs, whether that is a subscription, a spread on execution or something else, before wiring exchange credentials into it.

• It consolidates order books, positions and profit and loss from multiple prediction-market venues into a single blotter, cutting the tab-switching that comes with trading Polymarket and Kalshi separately.
• It runs execution algorithms such as icebergs and pegs on River's own infrastructure rather than the venue's, so cancelling a parent order unwinds every child leg at once.
• It signs API requests with a key that never leaves the trader's machine, which limits what a compromised server could do with a stolen credential.
• Subaccounts isolate strategies and credentials from one another, so a problem with one book does not automatically expose another.
• Its smart order router checks connected venues for the best price on a matched contract before committing size to a single book.

• It publishes no fee schedule anywhere in its own documentation, leaving a trader to estimate the cost of routing size through it only by asking directly.
• It markets itself as infrastructure for hedge funds and prop desks, yet the fills it routes average $395, a size closer to an active individual account than an institutional block.
• 3 traders route through it on an average day and 14 wallets make up its entire thirty-day cohort, a narrow base of accounts rather than a broad crowd.
• It has 140 of history since its first attributed fill, not yet long enough to have been tested across a full market cycle.
• Trading through it means managing subaccounts, API keys and a Python SDK, a heavier setup than connecting a browser wallet directly to Polymarket.
• Novig is still listed as coming soon and Rothera as a future integration, so the multi-venue promise is not fully live yet.

River Markets holds a verified profile on Polymarket, and it encrypts the credentials it stores for a connected exchange account rather than keeping them in plain text, deleting them the moment a subaccount is removed. API requests are signed locally with a key the operator says never leaves the trader's machine, so its server only ever sees a public key. It publishes no fee schedule anywhere in its own documentation, so cost is something a trader has to ask about directly rather than read off a stated rate. Funds themselves stay in the underlying Polymarket or Kalshi account a subaccount connects to, not in River Markets.

River Markets reads as infrastructure for traders who already work prediction markets professionally, not a discovery tool for someone deciding what to bet on. The multi-book blotter, native execution algorithms and subaccount isolation solve real problems for a desk running size across Polymarket, Polymarket US and Kalshi at once, and locally-signed API keys are a reasonable answer to the custody question any trading API raises. What it does not solve is pricing: with no fee schedule published anywhere in its own pages, a trader has to ask before wiring in capital. That makes it a fit for prop desks and algorithmic traders, and a poor fit for someone placing a single casual bet.

• What is River Markets?

  River Markets is a prime brokerage execution terminal for prediction markets, connecting one account to Polymarket, Polymarket US and Kalshi with a multi-book blotter, native execution algorithms and unified positions and profit and loss.

• Is River Markets safe to connect to Polymarket?

  It encrypts the exchange credentials it stores and signs API requests locally with a key that the operator says never leaves the trader's device, though it publishes no independent audit confirming either claim.

• What does River Markets cost?

  Its own site and documentation publish no fee schedule for the terminal, execution or SDK access, so a trader has to ask River directly what a subaccount costs before connecting one.

• Does River Markets hold my funds?

  No custody claim appears anywhere in its own pages; positions and balances stay in the underlying Polymarket or Kalshi account that a subaccount is wired to, not with River Markets.

• How does River Markets route orders to Polymarket?

  Orders reference a single River ID, and River's smart order router matches the same contract across every connected venue and sends it to whichever book is quoting the best price.

• What are the alternatives to River Markets?

  Predicts.guru tracks other Polymarket builders covering copy trading, mobile terminals and bot-driven order flow; compare #72 and routed volume against those before choosing where to send an order.

• Who is River Markets built for?

  Its own site names hedge funds, proprietary trading firms and professional traders as the intended audience, not casual bettors placing a single wager on one market.