diff --git a/src/include/utils/innerjoin.h b/src/include/utils/innerjoin.h
new file mode 100644
index 0000000..754982e
--- /dev/null
+++ b/src/include/utils/innerjoin.h
@@ -0,0 +1,36 @@
+#pragma once
+#include <iostream>
+#include <ostream>
+#include <stdlib.h>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+namespace buzzdb {
+namespace utils {
+
+class Table : public std::vector<std::vector<std::string>> {
+public:
+  Table() : std::vector<std::vector<std::string>>() {}
+  Table(std::initializer_list<std::vector<std::string>> list)
+      : std::vector<std::vector<std::string>>(list) {}
+
+  friend std::ostream &operator<<(std::ostream &os, const Table &table);
+};
+
+class InnerJoin {
+public:
+  InnerJoin(Table &left, Table &right, size_t left_attr_index,
+            size_t right_attr_index);
+  ~InnerJoin();
+
+  friend std::ostream &operator<<(std::ostream &os, const Table &table);
+
+  Table joined_table;
+
+private:
+  std::unordered_map<std::string, std::vector<std::string>> hashmap;
+};
+
+} // namespace utils
+} // namespace buzzdb
\ No newline at end of file
diff --git a/src/utils/innerjoin.cc b/src/utils/innerjoin.cc
new file mode 100644
index 0000000..34ee2cc
--- /dev/null
+++ b/src/utils/innerjoin.cc
@@ -0,0 +1,39 @@
+#include "../include/utils/innerjoin.h"
+#include <iostream>
+#include <string>
+#include <vector>
+
+#define UNUSED(p) ((void)(p))
+
+namespace buzzdb {
+namespace utils {
+InnerJoin::InnerJoin(Table &left, Table &right, size_t left_attr_index,
+                     size_t right_attr_index) {
+  for (size_t i = 0; i < left.size(); i++) {
+    hashmap[left[i][left_attr_index]] = left[i];
+  }
+
+  for (auto &tuple : right) {
+    if (hashmap.find(tuple[right_attr_index]) != hashmap.end()) {
+      std::vector<std::string> row = hashmap[tuple[right_attr_index]];
+      row.insert(row.end(), tuple.begin(), tuple.end());
+
+      joined_table.push_back(row);
+    }
+  }
+}
+
+InnerJoin::~InnerJoin() {}
+
+std::ostream &operator<<(std::ostream &os, const buzzdb::utils::Table &table) {
+  for (size_t i = 0; i < table.size(); i++) {
+    for (size_t j = 0; j < table[i].size(); j++) {
+      os << table[i][j] << '\t';
+    }
+    os << std::endl;
+  }
+  return os;
+}
+
+} // namespace utils
+} // namespace buzzdb
\ No newline at end of file
diff --git a/test/unit/utils/innerjoin_test.cc b/test/unit/utils/innerjoin_test.cc
new file mode 100644
index 0000000..ed6426b
--- /dev/null
+++ b/test/unit/utils/innerjoin_test.cc
@@ -0,0 +1,48 @@
+#include "utils/innerjoin.h"
+#include <algorithm>
+#include <gtest/gtest.h>
+#include <iostream>
+#include <string>
+
+namespace {
+using buzzdb::utils::InnerJoin;
+using buzzdb::utils::Table;
+
+Table relation_students{
+    // ID, Name, Age, Major
+    {"1", "Alice", "19", "CS"},   {"2", "Jonah", "20", "EE"},
+    {"3", "Charlie", "19", "CS"}, {"4", "David", "22", "EE"},
+    {"5", "Eve", "18", "CS"},
+};
+
+Table relation_grades{
+    // ID, Course, Grade
+    {"1", "CS101", "A"},
+    {"1", "EE200", "B"},
+    {"2", "CS101", "C"},
+    {"4", "EE200", "B"},
+};
+
+TEST(AlgorithmTest, InnerJoin) {
+  const Table expected_table = Table{
+      // ID, Name, Age, Major, ID, Course, Grade
+      {"1", "Alice", "19", "CS", "1", "CS101", "A"},
+      {"1", "Alice", "19", "CS", "1", "EE200", "B"},
+      {"2", "Jonah", "20", "EE", "2", "CS101", "C"},
+      {"4", "David", "22", "EE", "4", "EE200", "B"},
+  };
+
+  InnerJoin join{relation_students, relation_grades, 0, 0};
+
+  EXPECT_EQ(join.joined_table, expected_table);
+
+  std::cout << "Expected table:" << std::endl << expected_table << std::endl;
+  std::cout << "Joined table:" << std::endl << join.joined_table << std::endl;
+}
+
+} // namespace
+
+int main(int argc, char *argv[]) {
+  testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}