diff --git a/src/Makefile.am b/src/Makefile.am
index 521687eb45..0466d961c3 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -103,6 +103,7 @@ BITCOIN_CORE_H = \
   fs.h \
   httprpc.h \
   httpserver.h \
+  index/txindex.h \
   indirectmap.h \
   init.h \
   interfaces/handler.h \
@@ -204,6 +205,7 @@ libbitcoin_server_a_SOURCES = \
   consensus/tx_verify.cpp \
   httprpc.cpp \
   httpserver.cpp \
+  index/txindex.cpp \
   init.cpp \
   dbwrapper.cpp \
   merkleblock.cpp \
diff --git a/src/index/txindex.cpp b/src/index/txindex.cpp
new file mode 100644
index 0000000000..27cf844ceb
--- /dev/null
+++ b/src/index/txindex.cpp
@@ -0,0 +1,157 @@
+// Copyright (c) 2017-2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <index/txindex.h>
+#include <init.h>
+#include <tinyformat.h>
+#include <ui_interface.h>
+#include <util.h>
+#include <validation.h>
+#include <warnings.h>
+
+template<typename... Args>
+static void FatalError(const char* fmt, const Args&... args)
+{
+    std::string strMessage = tfm::format(fmt, args...);
+    SetMiscWarning(strMessage);
+    LogPrintf("*** %s\n", strMessage);
+    uiInterface.ThreadSafeMessageBox(
+        "Error: A fatal internal error occurred, see debug.log for details",
+        "", CClientUIInterface::MSG_ERROR);
+    StartShutdown();
+}
+
+TxIndex::TxIndex(std::unique_ptr<TxIndexDB> db) :
+    m_db(std::move(db)), m_synced(false), m_best_block_index(nullptr)
+{}
+
+bool TxIndex::Init()
+{
+    LOCK(cs_main);
+
+    // Attempt to migrate txindex from the old database to the new one. Even if
+    // chain_tip is null, the node could be reindexing and we still want to
+    // delete txindex records in the old database.
+    if (!m_db->MigrateData(*pblocktree, chainActive.GetLocator())) {
+        return false;
+    }
+
+    CBlockLocator locator;
+    if (!m_db->ReadBestBlock(locator)) {
+        locator.SetNull();
+    }
+
+    m_best_block_index = FindForkInGlobalIndex(chainActive, locator);
+    m_synced = m_best_block_index.load() == chainActive.Tip();
+    return true;
+}
+
+bool TxIndex::WriteBlock(const CBlock& block, const CBlockIndex* pindex)
+{
+    CDiskTxPos pos(pindex->GetBlockPos(), GetSizeOfCompactSize(block.vtx.size()));
+    std::vector<std::pair<uint256, CDiskTxPos>> vPos;
+    vPos.reserve(block.vtx.size());
+    for (const auto& tx : block.vtx) {
+        vPos.emplace_back(tx->GetHash(), pos);
+        pos.nTxOffset += ::GetSerializeSize(*tx, SER_DISK, CLIENT_VERSION);
+    }
+    return m_db->WriteTxs(vPos);
+}
+
+void TxIndex::BlockConnected(const std::shared_ptr<const CBlock>& block, const CBlockIndex* pindex,
+                    const std::vector<CTransactionRef>& txn_conflicted)
+{
+    if (!m_synced) {
+        return;
+    }
+
+    const CBlockIndex* best_block_index = m_best_block_index.load();
+    if (!best_block_index) {
+        if (pindex->nHeight != 0) {
+            FatalError("%s: First block connected is not the genesis block (height=%d)",
+                       __func__, pindex->nHeight);
+            return;
+        }
+    } else {
+        // Ensure block connects to an ancestor of the current best block. This should be the case
+        // most of the time, but may not be immediately after the the sync thread catches up and sets
+        // m_synced. Consider the case where there is a reorg and the blocks on the stale branch are
+        // in the ValidationInterface queue backlog even after the sync thread has caught up to the
+        // new chain tip. In this unlikely event, log a warning and let the queue clear.
+        if (best_block_index->GetAncestor(pindex->nHeight - 1) != pindex->pprev) {
+            LogPrintf("%s: WARNING: Block %s does not connect to an ancestor of " /* Continued */
+                      "known best chain (tip=%s); not updating txindex\n",
+                      __func__, pindex->GetBlockHash().ToString(),
+                      best_block_index->GetBlockHash().ToString());
+            return;
+        }
+    }
+
+    if (WriteBlock(*block, pindex)) {
+        m_best_block_index = pindex;
+    } else {
+        FatalError("%s: Failed to write block %s to txindex",
+                   __func__, pindex->GetBlockHash().ToString());
+        return;
+    }
+}
+
+void TxIndex::SetBestChain(const CBlockLocator& locator)
+{
+    if (!m_synced) {
+        return;
+    }
+
+    const uint256& locator_tip_hash = locator.vHave.front();
+    const CBlockIndex* locator_tip_index;
+    {
+        LOCK(cs_main);
+        locator_tip_index = LookupBlockIndex(locator_tip_hash);
+    }
+
+    if (!locator_tip_index) {
+        FatalError("%s: First block (hash=%s) in locator was not found",
+                   __func__, locator_tip_hash.ToString());
+        return;
+    }
+
+    // This checks that SetBestChain callbacks are received after BlockConnected. The check may fail
+    // immediately after the the sync thread catches up and sets m_synced. Consider the case where
+    // there is a reorg and the blocks on the stale branch are in the ValidationInterface queue
+    // backlog even after the sync thread has caught up to the new chain tip. In this unlikely
+    // event, log a warning and let the queue clear.
+    const CBlockIndex* best_block_index = m_best_block_index.load();
+    if (best_block_index->GetAncestor(locator_tip_index->nHeight) != locator_tip_index) {
+        LogPrintf("%s: WARNING: Locator contains block (hash=%s) not on known best " /* Continued */
+                  "chain (tip=%s); not writing txindex locator\n",
+                  __func__, locator_tip_hash.ToString(),
+                  best_block_index->GetBlockHash().ToString());
+        return;
+    }
+
+    if (!m_db->WriteBestBlock(locator)) {
+        error("%s: Failed to write locator to disk", __func__);
+    }
+}
+
+bool TxIndex::FindTx(const uint256& txid, CDiskTxPos& pos) const
+{
+    return m_db->ReadTxPos(txid, pos);
+}
+
+void TxIndex::Start()
+{
+    // Need to register this ValidationInterface before running Init(), so that
+    // callbacks are not missed if Init sets m_synced to true.
+    RegisterValidationInterface(this);
+    if (!Init()) {
+        FatalError("%s: txindex failed to initialize", __func__);
+        return;
+    }
+}
+
+void TxIndex::Stop()
+{
+    UnregisterValidationInterface(this);
+}
diff --git a/src/index/txindex.h b/src/index/txindex.h
new file mode 100644
index 0000000000..3d42a89635
--- /dev/null
+++ b/src/index/txindex.h
@@ -0,0 +1,60 @@
+// Copyright (c) 2017-2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_INDEX_TXINDEX_H
+#define BITCOIN_INDEX_TXINDEX_H
+
+#include <primitives/block.h>
+#include <txdb.h>
+#include <uint256.h>
+#include <validationinterface.h>
+
+class CBlockIndex;
+
+/**
+ * TxIndex is used to look up transactions included in the blockchain by hash.
+ * The index is written to a LevelDB database and records the filesystem
+ * location of each transaction by transaction hash.
+ */
+class TxIndex final : public CValidationInterface
+{
+private:
+    const std::unique_ptr<TxIndexDB> m_db;
+
+    /// Whether the index is in sync with the main chain. The flag is flipped
+    /// from false to true once, after which point this starts processing
+    /// ValidationInterface notifications to stay in sync.
+    std::atomic<bool> m_synced;
+
+    /// The last block in the chain that the TxIndex is in sync with.
+    std::atomic<const CBlockIndex*> m_best_block_index;
+
+    /// Initialize internal state from the database and block index.
+    bool Init();
+
+    /// Write update index entries for a newly connected block.
+    bool WriteBlock(const CBlock& block, const CBlockIndex* pindex);
+
+protected:
+    void BlockConnected(const std::shared_ptr<const CBlock>& block, const CBlockIndex* pindex,
+                        const std::vector<CTransactionRef>& txn_conflicted) override;
+
+    void SetBestChain(const CBlockLocator& locator) override;
+
+public:
+    /// Constructs the TxIndex, which becomes available to be queried.
+    explicit TxIndex(std::unique_ptr<TxIndexDB> db);
+
+    /// Look up the on-disk location of a transaction by hash.
+    bool FindTx(const uint256& txid, CDiskTxPos& pos) const;
+
+    /// Start initializes the sync state and registers the instance as a
+    /// ValidationInterface so that it stays in sync with blockchain updates.
+    void Start();
+
+    /// Stops the instance from staying in sync with blockchain updates.
+    void Stop();
+};
+
+#endif // BITCOIN_INDEX_TXINDEX_H